Guide

What is a purchase order?

What a purchase order is, how it differs from an invoice, what goes on one, and how the PO-to-invoice process actually works when you bill a client with an AP department.

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A purchase order (PO) is a document a buyer issues to a seller that authorizes a specific purchase: what is being bought, the quantity, the agreed price, and the terms. It is created before the work or the goods are delivered, and it commits the buyer to pay once the seller fulfills it. The invoice comes later and references the PO. In short, the purchase order is the buyer saying "we approve this purchase," and the invoice is the seller saying "here is the bill for it." If your client has a procurement or accounts payable team, they will often require a PO number on your invoice before they will pay it.

Purchase order vs invoice

They are easy to confuse because they describe the same transaction, but they are issued by different parties at different points in time.

→Who creates it. The buyer creates the purchase order. The seller creates the invoice.
→When. The PO comes first, before delivery. The invoice comes after, once the work is done or the goods are shipped.
→What it does. The PO authorizes and commits the buyer to the purchase. The invoice requests payment for it.
→How they connect. The invoice quotes the PO number so the buyer's AP team can match the bill back to the approved order.

If you are also unclear on how an invoice differs from a receipt or a quote, the invoice vs receipt guide covers the rest of the document family.

What goes on a purchase order

A purchase order is structured so the seller and the AP team can both act on it without ambiguity. A complete PO includes:

•A unique PO number, the reference both sides use for everything that follows
•The buyer and seller details, including the legal billing entity
•Line items: a description of each good or service, quantity, and agreed unit price
•The total authorized amount, which is the ceiling the invoice should not exceed
•Delivery and payment terms, such as the expected delivery date and Net 30
•The issue date and, often, an internal cost center or project code

Your invoice does not need to repeat all of this, but it should mirror the PO number, the line items, and the agreed amounts closely enough that the AP team can match the two without a phone call.

How the purchase order process works

For a freelancer or agency billing a client with procurement, the cycle usually runs like this:

You agree on scope and price with the client, often via a quote or statement of work.
The client raises a purchase order internally. Their procurement team approves the spend and issues a PO with a number.
The client sends you the PO, or at least the PO number. This is your signal that the spend is approved and you are clear to start.
You do the work or deliver the goods.
You invoice, quoting the PO number and matching the PO's line items and amounts.
AP matches the invoice to the PO (and to a delivery record, if there is one) and releases payment.

The failure point for most small suppliers is step 5. You invoice without the PO number because the client never sent it, the invoice lands in AP, and it stalls. Getting the PO number onto the invoice is the single biggest thing you can do to avoid a payment delay with a procurement-driven client.

PO matching: why AP is strict about it

Accounts payable teams do not withhold your payment to be difficult. They are running a control called matching, and your invoice has to pass it.

→Two-way matching compares the invoice against the purchase order. The PO number, the line items, and the amounts have to line up.
→Three-way matching adds a third document, the goods receipt or proof of delivery, and checks all three agree.

If your invoice has no PO number, there is nothing to match against and it cannot move through an automated AP system at all. If the amount on your invoice exceeds the PO's authorized total, it fails the match and gets kicked back. The fix is simple: get the PO number before you invoice, and keep your invoice total at or under the PO amount. If scope grew, ask the client to raise a PO amendment or a new PO rather than quietly invoicing over the limit.

Handling PO numbers on your invoices

The awkward reality is that you often do not have the PO number when you are ready to invoice. The client's procurement team raised it, your day-to-day contact may not have passed it on, and chasing it turns into an email thread.

There are two clean ways to handle it:

•Ask for the PO number before you start the work. Make it part of your kickoff. A PO is the client confirming the spend is approved, so you want it anyway.
•Let the recipient add the PO number to the invoice themselves. JupiterInvoice's recipient editing lets the person who opens the invoice link add or correct the PO number directly, with no reissue and no email back-and-forth. The change is versioned and you are notified.

If you receive an inbound invoice and want to sanity-check a PO number's format, the PO number checker flags common formatting issues.

Purchase order FAQ

Does a small freelancer need to use purchase orders?

Not for most small or owner-operated clients, who simply approve a quote and pay the invoice. But the moment you bill a larger company with a procurement or accounts payable team, that client will often require a PO. The PO is their internal control, not yours, so you adapt to whether the client uses them.

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

The buyer issues the purchase order before delivery to authorize and commit to a purchase. The seller issues the invoice after delivery to request payment. The invoice references the PO number so the buyer can match the bill to the approved order.

Can I send an invoice without a PO number?

You can, but if the client's AP process requires a PO, an invoice without one cannot be matched and will stall. If a client uses POs, get the number before you invoice, or use an invoicing tool that lets the recipient add it to the invoice themselves.

What happens if my invoice is more than the purchase order amount?

It fails AP matching and gets returned. The PO total is a ceiling. If scope or hours grew beyond it, ask the client to raise a PO amendment or a new PO covering the extra amount, then invoice against the updated authorization. Do not invoice over the PO total and hope it slips through.

Who creates the purchase order, the buyer or the seller?

The buyer. The PO is the buyer's internal authorization to spend money with you. As the seller you receive it, reference its number on your invoice, and keep your invoice consistent with what the PO authorized.

Is a purchase order legally binding?

Once the seller accepts it, a purchase order generally forms a binding contract for that specific purchase: the buyer is committed to pay for what the PO covers, delivered as specified. The exact legal weight depends on your jurisdiction and any overarching master agreement, but in practice both sides treat an accepted PO as the deal.