Procurement basics
Purchase Order vs Invoice: Key Differences Explained
A purchase order and an invoice look similar on paper, but they play opposite roles. The buyer issues the purchase order to request goods or services. The seller issues the invoice to request payment. Get the distinction right and your procurement, accounting, and audit trail stay clean.
The one-sentence answer
A purchase order (PO) is a buyer's authorized request to purchase specific items at agreed prices. An invoice is the seller's request for payment after those items are delivered. The PO opens the transaction; the invoice closes it.
Side-by-side comparison
| Attribute | Purchase order | Invoice |
|---|---|---|
| Who issues it | Buyer (procurement or requester) | Seller (accounts receivable) |
| Purpose | Authorize and commit to a purchase | Request payment for delivered goods |
| Timing | Before fulfillment | After delivery or on agreed schedule |
| Legal role | Binding contract once accepted | Payment demand and tax record |
| Reference number | PO number (assigned by buyer) | Invoice number + referenced PO number |
| Tax handling | Estimated tax lines only | Final tax (Sales Tax, VAT, or GST) |
| Accounting impact | Commitment / encumbrance | Accounts payable liability |
| Typical fields | Vendor, ship-to, line items, agreed prices, delivery date | Bill-to, line items, tax, total due, payment terms, remit-to |
When to use a purchase order
Use a PO whenever your organization needs a documented, pre-approved commitment to buy. Common triggers:
- Recurring orders from an approved vendor.
- Purchases above an internal approval threshold.
- Multi-line orders where prices, quantities, and delivery dates must be locked in.
- Cross-border purchases that need clear Incoterms and currency.
- Any spend that must be tied to a budget line or project code.
When you receive an invoice
The seller issues the invoice once the ordered items ship or the service is performed. Accounts payable then runs a three-way match: the PO, the goods receipt, and the invoice must agree on quantities, prices, and totals before payment is released. A missing or mismatched PO number is the single most common cause of AP delays.
How the two documents flow together
- Requester raises a purchase request.
- Buyer issues a PO to the vendor with an assigned PO number.
- Vendor accepts the PO — it is now a binding contract.
- Vendor ships the goods or performs the service.
- Vendor issues an invoice that quotes the same PO number.
- Accounts payable performs a three-way match and pays per the invoice's terms.
Common mistakes to avoid
- Treating a quotation, order confirmation, or invoice as a PO. Only a document issued by the buyer counts.
- Paying an invoice that does not reference a valid PO — you lose your audit trail and negotiated pricing.
- Reusing a PO number across vendors or fiscal years.
- Omitting currency, Incoterms, or tax jurisdiction on cross-border POs.
Quick FAQ
Does a PO count as an invoice?
No. A PO commits the buyer to purchase; it does not demand payment. The seller still has to issue an invoice.
Which comes first?
The PO. Buyer issues the PO, seller fulfills, seller then invoices against the PO number.
Is a PO legally binding?
Once the seller accepts it, yes — it is a contract covering items, quantities, prices, and delivery terms.
Do small businesses really need POs?
For one-off retail purchases, no. For recurring vendors, approvals over a threshold, or anything you need to reconcile later, yes — POs are the cheapest audit trail you can create.