Purchase order vs invoice: the two documents, annotated

A purchase order is the document you send, saying "here's what I'm buying and at what price." An invoice is the document they send, saying "here's what you owe me." They describe the same transaction from opposite sides — and that's exactly why comparing them is the cheapest error-catching habit in procurement: any disagreement between the two is a mistake one of you just made.

The confusion is understandable because both list products, quantities, and prices. The difference is direction, timing, and legal weight:

The PO is the anchor. It's the document you control, written from your decisions. The invoice is a claim against it. When people talk about the purchase order process end to end, the PO is the spine every other document hangs from — confirmations, shipping notices, receipts, and finally the invoice that asks to be paid.

Document 1, annotated: the purchase order

Here's a real-shaped PO for a store buying camping stoves, with notes on what each field is for:

PURCHASE ORDER — INV-2027-041 From: Summit Outpost (your store) · To: Summitware Outfitting Co. (supplier) PO date: Oct 6, 2026 · Requested delivery: Nov 10, 2026

Line	SKU	Description	Qty	Unit price	Total
1	STV-PRP-2B	Propen 2-burner stove, case of 4	25 cases	$88.00	$2,200.00
2	STV-PRP-2B-GRP	Replacement grate (spare part)	40	$6.75	$270.00

Terms: Net 30 from invoice date. FOB our dock. Ship via LTL, liftgate required. Authorized by: (signature / name)

What each part does:

• The PO number — INV-2027-041 — is the thread you'll follow through every later document: their confirmation, the packing slip, the invoice, the payment. If you don't yet have a numbering convention, PO numbers explained covers formats and why the paper trail anchors on it.
• SKUs and per-line quantities. "25 cases of 4" is deliberately explicit. A PO that says "100 stoves" invites the supplier to decide what a case pack means; a PO with unit math doesn't.
• Unit price per line. This is the number the invoice will be checked against. If a price isn't on the PO, you can't object when the invoice "remembers" a different one.
• Terms line. Payment terms and shipping terms live here. If the invoice arrives with terms that contradict the PO, the PO (which you both agreed to) wins — that's the point of having it.

A proforma invoice, by the way, is a fourth document that looks like an invoice but isn't one — suppliers use it to quote a deal before it's final; proforma vs commercial invoice covers when they demand it (mostly for imports and letters of credit).

Document 2, annotated: the invoice

Three weeks later, the invoice arrives:

INVOICE #SW-88412 From: Summitware Outfitting Co. · Bill to: Summit Outpost Invoice date: Nov 4, 2026 Reference: Your PO INV-2027-041

Line	Item	Qty	Unit price	Total
1	Propen 2-burner stove, case of 4	25 cases	$88.00	$2,200.00
2	Replacement grate	40	$6.75	$270.00

Total due: $2,470.00 — Net 30 (due Dec 4, 2026)

What to notice — the three fields that matter:

1. The PO reference. A supplier invoice that cites your PO number is easy to verify. An invoice with no PO reference is how duplicate and mystery bills sneak into accounts payable — file those back for the PO number before anything else.
2. Quantities and prices per line. These are claims. In this example they match the PO, so the invoice can be paid on its due date. If line 2 had said $7.40, the invoice disagrees with the agreed price by $26.00 — small, but multiply by a year of orders and it's a silent margin leak.
3. The due date. Net 30 runs from invoice date per the PO's terms. Mark it when the invoice arrives, not when you open the mail — a late-dated payment sours a supplier relationship for no reason.

Why the pair matters more than either alone

Neither document protects you by itself. A PO alone doesn't prove anything shipped. An invoice alone proves only that the supplier says you owe money. The protection comes from the comparison — and from adding a third document, the goods receipt: what actually arrived at your door.

That trio — PO vs receipt vs invoice — is called a three-way match, and it catches the errors neither document pair can: the invoice that prices a shipment that arrived short, the PO you were billed for in full but received 90% of. If you only build one spreadsheet habit out of this article, make it that one; the walkthrough is in three-way matching.

What the PO vs invoice pair (a two-way match) still catches on its own:

• Price errors — the invoice "remembers" a different unit price than the quoted and PO'd one.
• Duplicate invoices — same goods, two invoice numbers, one PO reference.
• Terms drift — net 30 becomes net 15, or freight appears as a line item you never agreed to.
• Wrong-entity invoices — the bill comes from a differently-named sister company, which is also a mild fraud flag worth a verification email.

Where this happens in a small store, practically

You don't need accounts-payable software. In practice the loop is: PO drafted (native Shopify purchase orders can hold the record, as of October 2026 — see the Shopify Help Center's purchase orders documentation, accessed October 2026) → emailed to the supplier → packing slip checked at receiving → invoice arrives by email → you spend five minutes comparing invoice lines to PO lines and your receiving notes → pay on the due date, or dispute first.

The five-minute comparison is the whole discipline. Most small stores skip it and pay off the invoice total, which trains suppliers that invoice errors are your problem. Paying against the match — PO, receipt, invoice all agreeing, or a documented exception — is what keeps the paperwork honest for both sides.

Common questions

What is the difference between a purchase order and an invoice? A purchase order is created by the buyer before the transaction, committing to specific products, quantities, and prices. An invoice is created by the seller after shipping, requesting payment. The PO states what was agreed; the invoice states what the seller claims is owed — so the two should always match, and mismatches are errors to resolve before payment.

Does the invoice or the PO win in a disagreement? The PO generally wins, because it's the document both parties agreed to in advance. If an invoice's price or terms differ from the PO, the buyer's remedy is a short dispute email citing the PO number and line — not silent payment of the higher amount. Suppliers correct invoices against PO references routinely.

Should I pay an invoice without a PO number on it? Not before reconciling it. An invoice without a PO reference can't be matched to an agreed order — it might be a legitimate reorder you forgot to paper, a duplicate, or a bill for something you never ordered. Ask the supplier to tie it to a PO number (or explain the gap) before scheduling payment.

What comes first, PO or invoice? The PO, always. The sequence is: PO issued → supplier confirms → goods ship and arrive → invoice issued → you match invoice to PO and receipt → payment on terms. An "invoice" that arrives before any PO exists usually means the transaction was agreed verbally — paper it with a PO after the fact so the trail exists for the next dispute.

Is a proforma invoice the same as an invoice? No. A proforma is a preliminary quote document that looks like an invoice but requests no payment and creates no obligation — common in importing. The commercial invoice is the real billing document. They serve different steps of the same trail.

Next step: pull your three most recent supplier invoices and check each one's PO reference, line prices, and due date against your records. Whatever doesn't reconcile is your backlog; whatever reconciles is a habit worth keeping — upgrade it to the full three-way match when you're ready.