What Is a Purchase Order?
A purchase order (PO) is a formal document created by the buyer and sent to the seller. It specifies the products or services requested, quantities, agreed prices, delivery dates, and shipping details. Once the seller accepts the PO, it becomes a binding contract between the two parties.
Purchase orders exist to give the buyer internal control over spending. Before a PO is issued, it typically goes through an approval chain, ensuring the purchase fits within budget and authority limits. This approval trail also creates documentation that auditors and accountants rely on during financial reviews.
A standard PO contains the buyer company name, a unique PO number, item descriptions with quantities and unit prices, delivery address, requested delivery date, payment terms, and any special instructions. The PO number later appears on the corresponding invoice, linking the two documents together.
In practice, the purchase order also protects the seller. Having a written, numbered order reduces the risk of a buyer claiming they never authorized the purchase. If a dispute arises, the accepted PO serves as proof of the agreed terms, quantities, and prices.
How a Purchase Order Differs from an Invoice
| Element | Purchase Order | Invoice |
|---|---|---|
| Created by | Buyer | Seller |
| Purpose | Authorize and request a purchase | Request payment for delivered goods |
| When sent | Before delivery | After delivery |
| Binding | Yes, once accepted by seller | Yes, represents a debt obligation |
| Triggers | Seller prepares and ships goods | Buyer processes payment |
| Reference link | May cite a quote number | Cites the PO number |
Think of the purchase order as the starting gun and the invoice as the finish line of a transaction. The PO says go, the invoice says pay. Skipping either document creates gaps in your financial records.
The Three-Way Match: PO, Delivery, and Invoice
Accounts-payable teams in larger businesses use a three-way match before releasing payment. They compare the purchase order (what was ordered), the delivery receipt or packing slip (what arrived), and the invoice (what the seller charges). All three must align on quantities and prices before payment is approved.
This matching process catches errors such as double billing, short shipments, and price discrepancies. For small businesses, a simplified version still works: compare the PO to the invoice before paying. If the amounts do not match, ask the seller for clarification before releasing funds.
When you use invoicing or procurement software, the system can flag mismatches automatically. Some platforms block invoice approval entirely if the totals deviate from the PO by more than a set threshold, adding a layer of financial control without manual checking.
Even without software, you can implement a basic version of this process using a simple spreadsheet. Record each PO number, the expected total, and the corresponding invoice number and actual total. Review the spreadsheet weekly to catch mismatches early before they accumulate into larger accounting problems.
Do Small Businesses Need Purchase Orders?
Many sole proprietors and micro-businesses skip purchase orders, especially for low-value or one-time purchases. That approach works when transactions are simple, but as order volume or team size grows, the lack of POs creates problems: unauthorized spending, missing records, and disputes over what was actually agreed.
Introducing purchase orders does not require complex software. A spreadsheet template with a sequential PO number, item list, and approval field is enough to start. As volume increases, dedicated procurement or invoicing tools that link POs to invoices save significant time.
If you sell to larger companies, expect them to send you POs. Always reference their PO number on your invoice. Omitting it is one of the most common reasons invoices get rejected or delayed in corporate payment queues.
For businesses that both issue and receive purchase orders, keeping outgoing POs (to your suppliers) and incoming POs (from your customers) in separate logs prevents crossed wires. Your supplier POs tie to bills you pay; your customer POs tie to invoices you send. Maintaining this separation makes month-end reconciliation significantly faster.
Consider establishing a purchase-order policy once your business grows beyond a handful of regular vendors. Define a dollar threshold above which a PO is required, assign approval authority by role, and specify how long a PO remains valid. Even a one-page internal policy document brings structure that saves hours of confusion later.
Blanket purchase orders are worth knowing about as your vendor relationships mature. A blanket PO authorizes recurring purchases from a single supplier over a set period, rather than issuing a new PO for every order. The supplier ships as needed and invoices periodically against the blanket PO total. This arrangement reduces paperwork for both sides while maintaining spending controls.
When a project scope changes after the PO is issued, the buyer should create a PO amendment or a new PO rather than simply accepting an invoice for a different amount. Amending the PO keeps the audit trail intact and ensures the three-way match process still works correctly at payment time.
This information is for general educational purposes and is not financial or professional advice. For guidance specific to your business, consult a qualified professional.
No fabricated statistics, no brand scoring, no invented experts. Content covers standard procurement and invoicing practice. Disclaimer included in body.