Purchase Order vs Invoice: Key Differences Explained

A purchase order and an invoice both list goods, prices, and a vendor, so it is easy to treat them as two names for the same paperwork. They are not. One is created by the buyer to order something; the other is created by the seller to ask for payment. Confusing them is how AP teams end up paying for goods they never received, or paying the wrong amount. This guide explains exactly what each document is, which comes first, how they differ, and how your accounts payable process uses both to make sure you only pay for what you actually agreed to buy.

What is the difference between a purchase order and an invoice?

The difference between a purchase order and an invoice is who creates it and why. A purchase order (PO) is issued by the buyer to request goods or services and commit to buying them at a set price. An invoice is issued by the seller after delivery to request payment for those goods or services. The PO starts the transaction; the invoice ends it.

Think of the PO as the buyer saying "please send me these 200 units at this price," and the invoice as the seller replying "here is what I delivered, now please pay this amount by this date." They often look similar because they describe the same order, but they sit on opposite sides of the deal and serve opposite purposes: one authorizes a purchase, the other collects on it.

Which comes first, a purchase order or an invoice?

The purchase order always comes first. The buyer creates and sends the PO to order goods or services before anything is delivered or paid. The seller fulfills the order, then sends an invoice afterward to request payment. So the normal sequence is PO, then delivery, then invoice, then payment.

This order matters because it is what lets accounts payable verify a bill. When the invoice arrives, AP already has the PO on file showing what was agreed and at what price. Without that earlier document, there is nothing to check the invoice against, which is why purchase orders exist in the first place: they create the benchmark the invoice has to match.

What is the difference between a purchase order, an invoice, and a receipt?

A purchase order is the buyer's request to order goods, an invoice is the seller's request for payment, and a receipt (or delivery note) is proof the goods were actually received or that payment was made. The three documents cover the full lifecycle: ordering, billing, and confirming. Each is created at a different point by a different party.

In accounts payable these three are the backbone of verification. The PO says what you ordered, the receiving document (often called a goods receipt or packing slip) says what arrived, and the invoice says what you are being billed. Comparing all three before payment is the heart of three-way matching, and getting them to line up is what stops overbilling and fraud. Our guide to three-way matching in accounts payable walks through exactly how the comparison works.

Do you need both a purchase order and an invoice?

For most B2B purchases, yes. The purchase order documents what you agreed to buy and the invoice documents what you are being billed, and matching the two is how you confirm a charge is correct before paying. Small or recurring purchases, like a monthly software subscription or a utility bill, often skip the PO and run on the invoice alone.

Whether you use both usually depends on the spend. High-value or one-off orders almost always start with a PO so there is a record to check against. Routine, low-risk bills are handled as non-PO invoices, approved by whoever owns that budget. Many AP teams set a dollar threshold: above it, a PO is required; below it, an invoice plus an approver is enough. How those two paths are handled is a core part of any accounts payable process flow.

What is a PO number on an invoice?

A PO number on an invoice is the reference code from the original purchase order that the seller prints on their bill so it can be matched back to the order. When AP receives the invoice, the PO number tells them which purchase it relates to, so they can pull the matching PO and confirm the prices, quantities, and terms line up before paying.

The PO number is the link that makes matching possible at scale. Without it, an AP clerk has to guess which order an invoice belongs to, which is slow and error-prone. That is also why a missing or wrong PO number is one of the most common reasons an invoice gets flagged as an exception and sent back for rework, rather than flowing straight through to payment.

Is a purchase order legally binding?

A purchase order becomes legally binding once the seller accepts it. On its own, a PO is an offer to buy. When the vendor confirms or begins fulfilling the order, it forms a contract covering the items, quantities, prices, and terms listed. An invoice is also legally binding, but as a request for payment under terms that were already agreed.

This is one of the most useful practical differences. Because the accepted PO is a contract, it sets the price and quantity the seller is allowed to bill. If an invoice later shows a higher price or more units than the PO authorized, the buyer has documented grounds to dispute it. The PO is not just paperwork; it is the leverage that keeps the final bill honest.

What is the difference between a proforma invoice and a purchase order?

A proforma invoice is a preliminary bill the seller sends before delivery to show the buyer expected costs, while a purchase order is the buyer's formal commitment to buy. The proforma comes from the seller and is an estimate or quote; the PO comes from the buyer and is a binding order. They move in opposite directions even though both appear early in a deal.

The two are easy to mix up because both can arrive before any goods ship. The simplest test is direction: if the buyer issued it to order something, it is a purchase order; if the seller issued it to outline costs before a real invoice, it is a proforma. A proforma is never recorded in accounts payable as a payable, because it is not yet a demand for payment.

How do purchase orders and invoices work together in accounts payable?

In accounts payable, the purchase order sets the expectation and the invoice is checked against it before payment. When a vendor's invoice arrives, AP matches it to the open PO and the receiving record to confirm the price, quantity, and terms all agree. If everything matches, the invoice is approved and paid; if not, it becomes an exception to investigate.

That matching step is where the two documents earn their keep, and it is also where most AP time disappears, because someone has to read both documents and compare them field by field. The slow part is rarely the decision; it is getting the numbers off the PDF in the first place. Teams that automate the comparison move to an accounts payable automation platform that matches PO to invoice automatically, while teams that work in spreadsheets first need the invoice as clean, structured data they can line up against the PO. Procurement and AP teams handling this regularly can go deeper in our guide to invoice processing for procurement managers.

Turn invoices into data you can match

Matching an invoice to its purchase order only works when you can read the invoice as structured numbers, not a flat PDF. InvoiceXLSX does that step: upload a PDF or image invoice and get clean Excel or CSV with the vendor, PO number, dates, totals, and every line item already separated into columns. From there it takes seconds to look up the PO number and compare prices and quantities, instead of squinting at two documents side by side. You can extract invoice data to Excel, capture full invoice line item data for line-level matching, or run the whole batch through AI invoice data extraction so the matching step starts with accurate data every time.