What Is a Purchase Order? When It Becomes a Contract, and Where to Set Your PO Threshold

What is a purchase order? The short answer

A purchase order (PO) is the document a buyer sends a vendor to say exactly what it wants to buy: the items or services, quantities, agreed prices, delivery date and payment terms, under a unique PO number. Legally it is an offer. Once the vendor accepts it, by confirming or simply by shipping, it becomes the contract both sides are held to, and later it is the yardstick your accounts payable team checks the vendor's invoice against.

As of October 2026, the useful questions for a small business are not "what is a PO" but when you need one, what the vendor's acknowledgment can quietly change, and where to set the dollar line below which nobody bothers. This guide answers those with the actual rules: the Uniform Commercial Code that governs sales of goods in every state, and the Federal Acquisition Regulation, which publishes the purchasing thresholds the U.S. government uses on itself.

What goes on a purchase order

The payment terms on the PO are what start the clock on when the invoice is due. If net 30 versus net 30 EOM is fuzzy, our guide to net 30 payment terms works through the due-date math.

What is a PO number, and how should you number them?

A PO number is the identifier you assign to a purchase order so that every document in the transaction can be matched to it. Vendors are asked to print it on their invoice, and many larger customers reject invoices that arrive without one. That is why a vendor asks you "what's your PO number?" before they will bill you.

Any scheme works if it is unique and never reused. Three that small businesses use:

• Sequential: 10001, 10002, 10003. Simple, and gaps show you a PO was voided.
• Prefixed by year: 2026-0147. Easy to sort and to see when a PO is stale.
• Prefixed by department or job: SVC-2026-0147 or JOB-1182-03. Useful when you need to cost purchases to a job or location.

Whatever you pick, generate it in one place (your accounting or AP system), not in a spreadsheet three people edit.

Purchase requisition vs purchase order

A purchase requisition is internal: an employee asking permission to buy something. A purchase order is external: the approved request, sent to the vendor. The requisition never leaves the building; the PO does. In a small company the two are often one step (the owner approves and sends the PO directly). They split once someone other than the person buying controls the budget.

When a purchase order becomes a binding contract

This is the part most PO explainers skip, and it is where the money is. Sales of goods between U.S. businesses run on Article 2 of the Uniform Commercial Code, the model sales law the states enact as their own. Three sections do the work:

• The PO is an offer, and shipping accepts it. UCC 2-206 says an "order or other offer to buy goods for prompt or current shipment" invites acceptance "either by a prompt promise to ship or by the prompt or current shipment." So a vendor that never signs anything has still accepted your PO the day the goods leave its dock.
• The vendor's acknowledgment can add terms. Under UCC 2-207, a vendor's acceptance works "even though it states terms additional to or different from those offered." Between merchants those additional terms become part of the contract unless your offer "expressly limits acceptance to the terms of the offer," the terms "materially alter it," or you object within a reasonable time. This is the battle of the forms: your PO says one thing, their acknowledgment says another, and the fine print on the acknowledgment can stick if you said nothing.
• $500 is the paperwork line. UCC 2-201 makes a contract for the sale of goods "for the price of $500 or more" unenforceable without "some writing sufficient to indicate that a contract for sale has been made," signed by the party being held to it. A PO the vendor accepts in writing is exactly that writing.

The practical takeaway: put one sentence on every PO that limits acceptance to your terms, and read vendor acknowledgments for changed prices, delivery dates or liability limits before goods ship. A PO that nobody compares to the acknowledgment is a contract you did not read.

The finding: where to set your PO threshold, using the government's own numbers

Requiring a PO for every purchase buries a small team in paperwork. Requiring none means the first time you see a purchase is when the invoice arrives. The question is where the line goes, and the federal government publishes its answer. Under FAR 2.101 (read October 4, 2026):

• The micro-purchase threshold is $15,000. Below it, FAR 13.201 makes the government purchase card "the preferred method to purchase and to pay," with no formal competition required.
• The simplified acquisition threshold is $350,000. Between the two, agencies use simplified procedures; above it, full contracting rules apply.
• Construction is the exception: the micro-purchase line drops to $2,000 for construction covered by the federal prevailing-wage law.

Translated to a private business, that is a three-tier policy, scaled down to your size:

The small-business column is an example, not a rule: the $500 lower line borrows the UCC's writing threshold, and the upper line is a judgment about how much you would hate to lose. The structure is the part worth copying. One more lesson from the federal list: the government sets its construction line at $2,000, far below its general one, because that is where scope creep and change orders live. A contractor or property manager buying materials should set its PO line lower than an office buying software.

How the PO is used after the order: matching the invoice

The PO's second life starts when the invoice arrives. Accounts payable compares invoice to PO (a two-way match: did they bill the price we agreed?), and for physical goods also to the receiving record (a three-way match: did we get what we are paying for?). Our guide to the three-way match walks through a worked example where 20 missing units are caught before an $80 overpayment goes out. Without a PO, there is nothing to match against, and the invoice is approved on trust.

Two PO variants matter here:

• Blanket PO: one PO covering repeated purchases from a vendor over a period, up to a dollar cap (for example, 12 months of janitorial supplies up to $6,000). Each delivery is a release against it, and AP tracks the remaining balance.
• Standing or recurring PO: the same quantity at the same price on a schedule, such as a monthly service. These are the easiest to automate because every invoice should look the same.

Paper, spreadsheet or software

You can run POs from a numbered template, but the failure points are predictable: duplicate PO numbers, approvals done by text message, and nobody closing POs after the final invoice, so the open balance is wrong. Most accounting and AP tools now generate POs and carry the PO number onto the bill; what varies is whether approval routing and matching are in the plan you are quoting. Our roundup of the best accounts payable automation software compares which tools include PO matching, and the BILL plan lineup shows how often it is a top-tier feature.

The slow part is not creating the PO; it is reading every invoice and acknowledgment against it. That is the step our guide to automating accounts payable with AI breaks down: extraction and matching by software, exceptions by a person.

Let software read the paperwork

A PO process breaks quietly when the data is typed by hand: a PO number keyed wrong, a changed price on an acknowledgment nobody read, an invoice matched to the wrong order. SuperDupr's AI document processing reads POs, vendor acknowledgments, packing slips and invoices, flags any acknowledgment that changes your terms, matches invoices to the right PO, and sends only the real exceptions to your team.

The bottom line

A purchase order is your written offer to buy, and it turns into the contract the moment the vendor ships or confirms. It earns its keep twice: once by making someone approve the spend, and again when the invoice has to match it. Set a dollar line below which nobody writes one (the federal government's is $15,000; yours is probably closer to $500), put a limit-of-acceptance sentence on every PO, and read the vendor's acknowledgment before the goods arrive.

Sources (read October 4, 2026): Federal Acquisition Regulation 2.101, definitions of "micro-purchase threshold" and "simplified acquisition threshold," and FAR 13.201 (acquisition.gov); Uniform Commercial Code 2-201, 2-206 and 2-207 (Cornell Law School Legal Information Institute, law.cornell.edu).