Invoice vs. Purchase Order: Simple Guide

Invoice vs. purchase order at a glance
What goes on each one

A purchase order (PO) and an invoice describe the same sale from opposite ends. The buyer sends the purchase order before delivery, to order goods or services at agreed prices. The seller sends the invoice after delivery, to ask for payment, and quotes the PO number on it so the buyer can match the two. This page compares the two documents side by side. For the purchase order on its own, including whether it's a contract, see what is a purchase order.

Billed publishes this page.

Invoice vs. purchase order at a glance

Who sends it: the buyer sends the purchase order. The seller sends the invoice.
When: the purchase order goes out before the goods or services are delivered. The invoice goes out after delivery, or on the billing schedule both sides accepted.
What it does: the purchase order places and approves the order. The invoice asks for payment for what was delivered.
Its number: the purchase order has a PO number. The invoice has its own invoice number and quotes the PO number.
Its dates: the purchase order has an order date and a delivery date. The invoice has an invoice date and a due date.

What goes on each one

The purchase order lists the items or services, the quantities and unit prices, the delivery details and the terms. What is a purchase order lists each part.

The invoice bills for what was delivered. The OpenStax textbook Principles of Accounting, Volume 1 (section 12.1, opened October 30, 2026) describes an invoice from a supplier "detailing the purchase, credit terms, invoice date, and shipping arrangements". For the full list of what to put on an invoice, see how to create a professional invoice. For the terms, see invoice payment terms explained.

How the PO number links them

The buyer compares the invoice with the purchase order before paying: the items, the quantities, the unit prices and the total. The PO number on the invoice tells the buyer which order to check it against. If the invoice bills more than the purchase order covers, the buyer can hold the invoice until the difference is sorted out. The sample below shows one way that happens and how it's fixed.

If the client's order changes after the purchase order is sent, ask for a revised purchase order before you invoice the change.

When there's no purchase order

Not every client sends purchase orders. Without one, an approved estimate or a written order from the client records what was agreed. Quote its number or date on your invoice. See estimate vs quote vs proposal.

Keeping both documents (US)

The IRS recordkeeping page (last reviewed or updated May 1, 2026) says: "Purchases, sales, payroll, and other transactions you have in your business generate supporting documents. These documents contain information you need to record in your books." Keep each invoice with the purchase order it bills against. Ask your accountant how long to keep them.

Sample: an invoice that doesn't match the purchase order

These are sample figures for a fictional print shop and its client, a café.

Purchase order PO-2041 from the café:
- 500 flyers × $0.40 = $200.00
- 20 posters × $6.50 = $130.00
- PO total: $200.00 + $130.00 = $330.00
The change: the café asks by email for 100 more flyers, but PO-2041 isn't revised.
Invoice INV-1187 from the print shop, dated Friday, May 1, 2026, on net 30 terms, quoting PO-2041:
- 600 flyers × $0.40 = $240.00
- 20 posters × $6.50 = $130.00
- Invoice total: $240.00 + $130.00 = $370.00
The café's match: $370.00 − $330.00 = $40.00 over the PO. That's the 100 extra flyers: 100 × $0.40 = $40.00. The café holds the invoice.
The fix: the café revises PO-2041 to 600 flyers, which makes its total $370.00, the same as the invoice. The invoice is then approved, and payment is due on May 1 + 30 days = Sunday, May 31, 2026.

Had the print shop asked for the revised purchase order before invoicing, the invoice would have matched on the first check.

Where Billed fits

Billed's purchase order template page lists, among what the template includes, "Unique PO number for two-way matching against the invoice" and "References that carry through to the supplier's invoice".
Its invoice software page says: "Talk to your client about an invoice in one place, without leaving your dashboard."

When this guide isn't for you

If you need to know whether a purchase order binds you, see what is a purchase order and ask a lawyer where needed. If a client asks you to submit invoices through its own supplier portal, follow its instructions. To compare an invoice with other documents, see invoice vs. receipt, invoice vs. statement and invoice vs. bill.

Frequently Asked Questions

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

The buyer sends a purchase order before delivery, to order goods or services at agreed prices. The seller sends an invoice after delivery, to ask for payment, and quotes the PO number on it.

Which comes first, the purchase order or the invoice?

The purchase order. The invoice follows delivery, or the billing schedule both sides accepted, and refers back to the purchase order by its number.

What happens if an invoice doesn't match the purchase order?

The buyer can hold the invoice until the difference is sorted out. In the sample on this page, a $370.00 invoice against a $330.00 purchase order is held until the café revises the order to cover the extra 100 flyers.

Do I need a purchase order to send an invoice?

Only if your client's process requires one. Without a purchase order, quote the approved estimate or the client's written order on your invoice.

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