Accurate as of 13 November 2026.
TL;DR
Every sale and every purchase leaves a trail of documents. On the selling side it usually runs quotation → sales order → delivery order → invoice → statement → receipt. On the buying side it runs purchase order → goods received note → supplier invoice → payment.
Each document proves one thing. A quotation proves the price you offered. A delivery order proves the goods arrived. An invoice is the bill. Mixing them up is where disputes start.
Only bills for payment count as invoice data for GST. Under IRAS rules, sales orders, pro forma invoices and statements of account do not trigger the time of supply, and they are not sent to IRAS under GST InvoiceNow. Tax invoices, credit notes and debit notes are.
Never edit an invoice you've already issued. Use a credit note to reduce it or a debit note to increase it.
Year-end is when weak trails show. Auditors match delivery orders to invoices to check that sales land in the right year.
Free assets below: a document map, a GST and InvoiceNow table, a which-document-to-issue guide and an 8-point Document Trail Test.
It's the second week of January. Your auditor sends over a list of 25 invoices and asks for the delivery order behind each one.
Invoice 1047 was for 40 cartons. The delivery order says 36. The signed copy is in the driver's folder, or in the van, or it never got signed. Nobody can remember whether the other four went out on 30 December or 2 January, and that decides which year the sale belongs to.
Nobody did anything wrong. The quotation was in an email. The order came in on WhatsApp. The delivery order was written by hand in the store. The invoice was typed up from memory two days later. Every document existed. They just didn't point to each other.
This guide explains what each everyday business document is for, who issues it, how they should connect, and which ones matter for GST. It ends with a 10-minute test you can run on your own records before year-end.
What is a business document trail?
A business document trail is the chain of documents that records a transaction from first offer to final payment. For a sale, that is usually a quotation, sales order, delivery order, invoice and receipt. For a purchase, it is a purchase order, goods received note, supplier invoice and payment. Each document refers to the one before it, so anyone can trace what was agreed, delivered, billed and paid.
The 11 documents at a glance
Save this table. It is the quickest way to settle "which document is this?" in the office.
Document | Issued by | When | What it proves | Bill for payment? |
|---|---|---|---|---|
Quotation | Seller | Before the order | The price and terms you offered | No |
Purchase order (PO) | Buyer | When the buyer decides to buy | What the buyer agreed to order, at what price | No |
Sales order (order confirmation) | Seller | When you accept the order | What you committed to supply | No |
Pro forma invoice | Seller | Before supply, often for a deposit | What the bill will look like | No |
Delivery order (DO) | Seller | When goods leave you | What was delivered, and (once signed) that it arrived | No |
Goods received note (GRN) | Buyer | When goods arrive | What the buyer actually received | No |
Invoice / tax invoice | Seller | When you bill | What the customer owes and why | Yes |
Credit note | Seller | After an invoice, to reduce it | A refund, return or discount | Adjusts a bill |
Debit note | Seller | After an invoice, to increase it | An extra amount owed | Yes |
Statement of account (SOA) | Seller | Usually monthly | What is still outstanding | No (a reminder) |
Receipt | Seller | When payment arrives | That you were paid | No (proof of payment) |
"Bill for payment" follows the way IRAS describes invoice documents in its GST guides. Some businesses also call the DO a delivery note; the job is the same.
The selling side: from quotation to payment
Quotation
A quotation is the seller's written offer: items, quantities, prices, validity period, delivery and payment terms. Its job is to fix the price before anyone commits. Give every quotation a number and an expiry date, so a customer can't hold you to last year's price twelve months later.
Sales order (order confirmation)
When a customer accepts, the seller raises a sales order. It records exactly what you've agreed to supply and should quote the customer's PO number. Internally, it is the instruction to the store or production team. It is not a bill: IRAS lists sales orders among documents that are not invoices for working out when GST is due.
Pro forma invoice
A pro forma invoice looks like an invoice but isn't one. It is used when a customer needs a document to arrange payment in advance, such as a deposit, or to support an import or a bank application. IRAS does not treat a pro forma invoice as an invoice for time-of-supply purposes. Don't record it as a sale. When the payment actually arrives, the normal GST rules apply to that payment, and a tax invoice follows.
Delivery order (DO)
A delivery order goes out with the goods and lists what was delivered. The customer signs or stamps it on receipt. That signed copy is your proof of delivery, and it is the document you will need if a customer later says "we never got it" or an auditor asks when a sale happened. A DO usually shows quantities, not prices.
Invoice and tax invoice
The invoice is the bill. If you are GST-registered and your customer is GST-registered, it must be a tax invoice with the particulars IRAS sets out, issued within 30 days of the time of supply. For sales up to S$1,000 including GST, you may issue a simplified tax invoice instead. Our tax invoice template guide covers the required details line by line.
The rule that saves the most arguments: invoice what was delivered, and quote the DO number on the invoice. If 36 cartons went out, the invoice says 36.
Statement of account (SOA)
A statement of account lists every invoice, credit note and payment for a customer, and the balance still owed. It is a reminder, not a new bill. Sending one every month is the cheapest collection habit there is. Our late payments playbook shows how to build a chasing routine around it.
Receipt
A receipt confirms you were paid. GST-registered businesses selling to non-GST-registered customers may issue a receipt instead of a tax invoice. If you do, IRAS requires it to be serially printed and to show your name, GST registration number, the date, the total including GST and the words "Price Payable includes GST". Keep a duplicate of every receipt you issue.
The buying side: from purchase order to payment
Purchase order (PO)
A purchase order is the buyer's written instruction to a supplier: what to supply, how many, at what price and by when. Raising a PO before you order, and having someone approve it, is the single most useful spending control a small business can have. It means every supplier bill has something to be checked against.
Goods received note (GRN)
When goods arrive, the person receiving them records what actually came in. That is the goods received note. It often differs from the PO: short deliveries, substitutions, damaged items. Recording it on the day is what stops you paying for stock you never got.
Supplier invoice and the three-way match
Three-way matching is the check that a supplier invoice agrees with both the purchase order and the goods received note before it is paid. The PO confirms the price and what was ordered, the GRN confirms what arrived, and the invoice is what the supplier is charging. If any of the three disagree, the bill is held until the difference is explained.
Most overpayments in small businesses are not fraud. They are a duplicate bill, a price that crept up, or a short delivery billed in full. A three-way match catches all three. For payments, add one more check: never change a supplier's bank details on the strength of an email. Our guide to supplier bank-change scams has a verification routine you can copy.
When things change: credit notes and debit notes
Once an invoice has been issued, it should never be edited or deleted. If the amount needs to change, you issue a new document that refers to the original.
Credit note: reduces what the customer owes. Use it for returns, damaged goods, an agreed discount after invoicing, or an overcharge. Quote the original invoice number and the reason.
Debit note: increases what the customer owes. Use it when you undercharged or extra charges apply. IRAS treats a debit note as a bill for payment, so it triggers GST in the same way an invoice does.
Editing the original instead leaves you with a customer holding one version, your ledger holding another, and a GST return that may already have used the first. A credit or debit note keeps both versions visible and explains the difference.
Which documents count for GST and GST InvoiceNow?
This is where the difference between "a document" and "a bill" matters. Under IRAS's general rule, the time of supply (when GST becomes due) is triggered by the earlier of two events: an invoice being issued or a payment being received. Under the GST InvoiceNow Requirement, what goes to IRAS is invoice data: the data on invoices or equivalent documents that serve as bills for payment.
Document | Does issuing it trigger the time of supply? | Sent to IRAS as invoice data under GST InvoiceNow? |
|---|---|---|
Quotation | No. It is an offer, not a request for payment | No |
Sales order | No. IRAS names it as not an invoice for this purpose | No. Listed by IRAS as not a bill for payment |
Pro forma invoice | No. IRAS names it as not an invoice for this purpose | No. Listed by IRAS as not a bill for payment |
Delivery order | No. It is not a bill (the general rule looks at invoice or payment) | Not listed by IRAS; it is not a bill for payment |
Tax invoice / simplified tax invoice | Yes | Yes |
Serially numbered receipt | The payment it records triggers it | Yes, treated as a simplified tax invoice transaction |
Debit note | Yes | Yes |
Credit note | Adjusts GST on the original supply | Yes |
Statement of account | No. IRAS names it as not an invoice for this purpose | No. Listed by IRAS as not a bill for payment |
Sources: IRAS e-Tax Guide "GST: General Guide for Businesses" (17th edition, 30 Jan 2026), paras 5.1.2 to 5.1.4 and 7.1 to 7.3; IRAS e-Tax Guide "Adopting GST InvoiceNow Requirement for GST-registered Businesses" (2nd edition, 9 Mar 2026), paras 3.3, 3.4 and 11.2. This table covers common standard-rated sales. Special schemes and situations have their own rules; check the guides or your tax adviser.
In practice, the documents IRAS cares about are the ones that bill. But the documents before them, the PO, the sales order and the DO, are what prove those bills are right. If you want the background on the network itself, start with what InvoiceNow is and how it works, and see what GST InvoiceNow means for purchase invoices.
Which document do I issue? A quick guide
Situation | Document to issue |
|---|---|
A prospect asks how much | Quotation, numbered, with a validity date |
The customer accepts or sends their PO | Sales order quoting their PO number |
The customer must pay a deposit before you start | Pro forma invoice for the deposit. Once the payment arrives, issue the tax invoice (or receipt) the GST rules call for |
Goods leave your store | Delivery order, signed or stamped by the customer on receipt |
Only part of the order is delivered | Delivery order for what went out; invoice what was delivered |
Goods are returned or you agree a discount after invoicing | Credit note quoting the original invoice and the reason |
You undercharged or extra charges apply | Debit note quoting the original invoice |
A customer asks what they still owe | Statement of account |
A walk-in customer pays S$300 in cash | If you're GST-registered: a serially printed receipt if the customer isn't GST-registered, or a simplified tax invoice (allowed up to S$1,000 including GST) if they are. If you're not GST-registered: an ordinary receipt |
You need stock from a supplier | Purchase order, approved before you send it |
Stock arrives from a supplier | Goods received note, recorded the same day |
A supplier bill arrives | Match it to the PO and GRN before it goes for payment |
The thread that holds it together: reference numbers
A trail only works if each document points back to the one before. The simplest way is to carry the previous number forward every time:
Q-1023 → SO-0871 (quotes customer PO 4500-118) → DO-0912 → INV-1047 → CN-0033 (if anything comes back)
With that thread in place, anyone can start from any document and find the rest in seconds. Without it, finding the DO behind an invoice means scrolling through a folder by date and hoping.
There is a second benefit. When each document is created from the one before, the lines carry over. Nobody re-types 40 cartons into four documents, so nobody types 36 into one of them by mistake.
Why the trail matters more in December
For businesses with a 31 December financial year end, the next few weeks decide how smooth the audit or year-end close will be. One of the first things accountants and auditors test is cut-off: whether goods delivered before year-end were invoiced in that year, and goods delivered after it were not. They do it by matching DO dates to invoice dates. A trail with missing or unsigned DOs turns a quick check into a week of emails. If you hold stock, pair this with our year-end stocktake checklist.
The trail is also your main defence against money leaking out unnoticed. The Association of Certified Fraud Examiners' Occupational Fraud 2026 study of 2,402 cases across 143 countries found a median loss of US$104,000 per case, with a typical scheme running about 12 months before it was found. A total lack of internal controls contributed to about a third of cases. Those are global figures, not a Singapore forecast. But the controls that would have helped are mostly the ones above: approved POs, recorded GRNs, matched bills and a second person on payments.
The 10-minute Document Trail Test
Pick three sales and three supplier bills from last month at random. For each one, see whether you can find every linked document in under two minutes. Then score your business on these eight points, one point each.
Every invoice quotes a DO or sales order number, and the DO can be found.
Every DO for delivered goods is signed or stamped by the customer, and the quantity matches the invoice.
Changes after invoicing are made with credit or debit notes. Issued invoices are never edited.
Pro forma invoices are kept apart and never counted as sales.
Every supplier bill is matched to a PO and a GRN before it is paid.
The person who approves a payment is not the person who raised the PO.
Customers with balances get a statement of account every month.
Records are kept for at least five years and can be found by document number.
7 to 8: your trail is in good shape. 4 to 6: expect questions at year-end; fix the gaps in the order listed. 0 to 3: the documents probably exist, but they live in different places. That is a systems problem more than a people problem.
Mistakes that cause most document arguments
Editing an issued invoice. Use a credit or debit note instead.
Treating a pro forma as a sale. It inflates revenue and confuses GST.
Unsigned or missing DOs. Without proof of delivery, "we never got it" is hard to answer.
Paying supplier bills without a PO or GRN to check against. Duplicates and price creep get through.
No reference numbers between documents. Every query becomes a search.
Typing the same lines into four documents. Each re-type is a fresh chance of a mismatch.
Accepting new bank details by email. Verify by phone on a number you already hold.
How AveriWorks keeps the trail connected
AveriWorks is cloud accounting and ERP software built for Singapore businesses. Each document is created from the one before, so the lines, prices and reference numbers carry forward on their own.
Free Starter plan: sales orders, invoices, credit notes, statements of account and supplier invoices on a double-entry ledger, with InvoiceNow and GST InvoiceNow included.
Quote-to-cash on paid plans: quotations become sales orders, delivery orders and invoices without re-typing.
Procure-to-pay on paid plans: purchase orders, goods received notes and supplier invoices, matched three ways before you pay.
One place to look: open any document and see the rest of its trail.
Plan contents are from the AveriWorks pricing page as of 13 November 2026. Check the pricing page for current plans and features.
Open a free Starter account, then run one real order from start to finish. You'll see within an afternoon whether your trail holds together. If you're weighing up how much system you need, read accounting software vs ERP first.
Frequently asked questions
What is the difference between a quotation and an invoice?
A quotation is an offer made before a sale. It states the price and terms the seller is willing to supply at and usually has an expiry date. An invoice is a bill issued after or at the point of supply, asking the customer to pay. A quotation creates no amount owing and does not trigger GST; an invoice does both.
What is a delivery order in Singapore?
A delivery order (DO) is the document that goes out with goods, listing the items and quantities delivered. The customer signs or stamps it on receipt, and the signed copy becomes the seller's proof of delivery. A DO is not a bill and usually shows no prices. The invoice should quote the DO number and bill the quantities on it.
Is a pro forma invoice a tax invoice?
No. A pro forma invoice is a preliminary document showing what a bill will look like, often used to collect a deposit. IRAS does not treat a pro forma invoice as an invoice for working out when GST is due, and lists it as not a bill for payment under GST InvoiceNow. A tax invoice is issued once the GST rules require one.
What is the difference between a credit note and a debit note?
A credit note reduces the amount a customer owes on an earlier invoice, for example after a return, a discount or an overcharge. A debit note increases it, for example after an undercharge or an extra charge. Both should quote the original invoice number and the reason, so the original invoice never has to be edited.
What is three-way matching?
Three-way matching is a check that a supplier invoice agrees with the purchase order and the goods received note before it is paid. The PO shows what was ordered and at what price, the GRN shows what arrived, and the invoice shows what is being charged. Any difference holds the payment until it is resolved.
Do quotations and delivery orders go to IRAS under GST InvoiceNow?
No. GST InvoiceNow covers invoice data, meaning documents that serve as bills for payment, such as tax invoices, simplified tax invoices, serially numbered receipts, debit notes and credit notes. IRAS lists sales orders, pro forma invoices and statements of account as not bills for payment. Quotations and delivery orders are not bills either.
How long should a Singapore business keep invoices and delivery orders?
IRAS requires businesses to keep their records for five years. That covers invoices, receipts, credit notes and the documents that support them. Keeping purchase orders, delivery orders and goods received notes for the same period means you can still prove what was ordered and delivered if a customer, supplier or IRAS asks.
Sources
IRAS, e-Tax Guide "GST: General Guide for Businesses", 17th edition, 30 January 2026 (time of supply, paras 5.1.2 to 5.1.4; tax invoices, simplified tax invoices and receipts, paras 7.1 to 7.3): https://www.iras.gov.sg/media/docs/default-source/e-tax/etaxguide_gst_gst-general-guide-for-businesses(1).pdf
IRAS, e-Tax Guide "Adopting GST InvoiceNow Requirement for GST-registered Businesses", 2nd edition, 9 March 2026 (scope of invoice data, paras 3.3, 3.4 and 11.2): https://www.iras.gov.sg/media/docs/default-source/uploadedfiles/gst/etaxguide_gst_invoicenow_requirement.pdf
IRAS, Record Keeping Guide for GST-registered Businesses, 12th edition, 30 January 2026: https://www.iras.gov.sg/media/docs/default-source/e-tax/record-keeping-guide-for-gst-registered-businesses.pdf
IRAS, Invoicing customers: https://www.iras.gov.sg/taxes/goods-services-tax-(gst)/basics-of-gst/invoicing-price-display-and-record-keeping/invoicing-customers
Association of Certified Fraud Examiners, Occupational Fraud 2026: A Report to the Nations: https://www.acfe.com/report-to-the-nations
Accounting Today, "Organizations lose 5% to fraud each year", 12 May 2026 (median loss and duration from the ACFE 2026 report): https://www.accountingtoday.com/news/organizations-lose-5-to-fraud-each-year
Brady Ware, "2026 ACFE Fraud Report Insights" (lack of internal controls, 33%): https://bradyware.com/2026-acfe-fraud-report-insights/
AveriWorks plan comparison: https://averiworks.com/pricing
Accuracy note
This article is accurate as of 13 November 2026. GST rules are summarised from the IRAS e-Tax Guides listed above, in the editions current on that date, and cover common standard-rated sales; special schemes and situations have their own rules. The ACFE figures are global and are cited to show how often weak controls play a part in fraud, not as a forecast for any Singapore business. Document numbers and examples are illustrative. AveriWorks plan features may change; check the pricing page for the current version. This article is general information, not tax, accounting or legal advice.