Invoice vs Purchase Order: Key Differences, Flow and Examples

Quick answer: In the invoice vs purchase order relationship, the purchase order (PO) comes first: the buyer sends it to the seller to order specific goods or services at agreed prices. The invoice comes last: the seller sends it after delivery to request payment for what was supplied. The PO authorizes a purchase; the invoice asks to be paid for it, and the two should match.

This guide compares both documents side by side, walks through the full PO-to-payment workflow, shows a worked example of invoicing against a PO with a partial delivery, and lists the mismatches that get invoices stuck. When you are ready to bill, our free invoice generator has a dedicated field for the client's PO number, and our guide to the PO number on an invoice explains where it should appear.

What is a purchase order?

A purchase order is a buyer-created document that tells a supplier exactly what the buyer wants to purchase. It is usually generated inside the buyer's procurement or accounting system after an internal approval, and it gives the supplier a reference number to quote on every later document.

A typical purchase order includes:

• A unique PO number and the date it was issued
• The buyer's legal name, billing address and delivery address
• The supplier's name and address
• Item descriptions, quantities and agreed unit prices
• Delivery dates, shipping terms and payment terms
• The name of the person who approved the purchase

For the buyer, a PO is a spending control. It means the cost was approved before anyone committed money. For the seller, an accepted PO is written confirmation of the order, which helps when a dispute arises later.

What is an invoice?

An invoice is a seller-created document that records a sale and requests payment. It lists what was delivered, how much is owed, the due date and how to pay. Once issued, it becomes an account receivable for the seller and an account payable for the buyer. If you are confused by the buyer-side terminology, read invoice vs bill.

Invoice vs purchase order: side-by-side comparison

Feature	Purchase order	Invoice
Created by	Buyer	Seller
Sent to	Seller	Buyer (usually accounts payable)
Timing	Before goods or services are supplied	After delivery, or at an agreed billing milestone
Purpose	Authorize and order a purchase	Request payment for a completed supply
Reference number	PO number, set by the buyer	Invoice number, set by the seller
Accounting effect	Usually none until goods are received (a commitment, not a liability)	Creates a receivable for the seller and a payable for the buyer
Tax role	Generally not a tax document	Often the tax document used to claim VAT, GST or sales tax records
Can be changed by	Buyer, by issuing a revised or amended PO	Seller, by issuing a credit note and a new invoice

The most useful row to remember is the last one. If quantities or prices change, the buyer amends the PO; the seller never edits an issued invoice but corrects it through a credit note.

How the purchase order to invoice workflow works

In a business that uses purchase orders, money moves through a predictable sequence. Knowing it helps you, as the seller, send an invoice that slots straight into the buyer's process.

1. Quote or estimate. The seller prices the work. See invoice vs quotation for how these documents differ.
2. Purchase requisition. Someone inside the buying company asks for approval to spend.
3. Purchase order issued. Once approved, the buyer sends a PO with a number, quantities and prices.
4. PO accepted. The seller confirms the order, ideally in writing, and flags any errors now rather than at invoice time.
5. Delivery. Goods ship with a packing slip or delivery note, or services are performed.
6. Goods received. The buyer records a goods received note confirming what actually arrived.
7. Invoice issued. The seller sends an invoice quoting the PO number, with lines that mirror the PO.
8. Matching and approval. Accounts payable checks the invoice against the PO and the receipt.
9. Payment. The buyer pays by the due date set in the payment terms.

Three-way matching: why your invoice must mirror the PO

Many finance teams use a control called three-way matching. Before paying, they compare three documents: the purchase order (what was ordered), the receiving record (what arrived) and the invoice (what the supplier is charging). If the quantity, unit price or total on the invoice doesn't agree with the other two, the invoice is held.

Check	Purchase order	Goods received	Invoice	Result
Item	Toner cartridge TC-90	Toner cartridge TC-90	Toner cartridge TC-90	Match
Quantity	200	120	200	Mismatch: billed for 80 units not yet received
Unit price	$42.00	—	$42.00	Match

In this case the seller invoiced the full order even though only 120 units had shipped. The invoice will sit in an exception queue until it is corrected. Some smaller buyers only do two-way matching (PO against invoice), but the lesson is the same: bill only for what was ordered and delivered.

Worked example: invoicing a partial delivery against a PO

Suppose a client sends purchase order PO-4471 for 200 toner cartridges at $42.00 each ($8,400.00) plus 40 hours of installation at $85.00 per hour ($3,400.00). The PO total is $11,800.00. You ship 120 cartridges in the first week, so your first invoice covers only those units.

Description	Qty	Rate	Amount
Toner cartridge TC-90 (PO-4471, line 1), partial shipment	120	$42.00	$5,040.00
Subtotal			$5,040.00
Sales tax (7%, example rate)			$352.80
Total due			$5,392.80

The remaining PO balance before tax is $11,800.00 − $5,040.00 = $6,760.00 (80 cartridges at $3,360.00 plus installation at $3,400.00). Your second invoice will cover those lines and reference the same PO. Tax rates vary by state and product, so check yours with our sales tax calculator rather than copying the example rate.

Notice how the description repeats the PO number and line reference. That small detail lets an accounts payable clerk match your invoice in seconds. An itemized invoice that mirrors the PO line by line is always faster to approve than a single lump-sum line.

Common mistakes when invoicing against a purchase order

• Missing or wrong PO number. The single most common reason PO-backed invoices bounce.
• Billing more than the PO value. Extra work needs a revised PO from the buyer before you invoice it.
• Different prices from the PO. If you raised a price after the PO was issued, it will not be paid until the PO is updated.
• Invoicing before delivery is recorded. If the receiving step hasn't happened, the match fails.
• Billing the wrong legal entity. Use the exact buyer name on the PO, not a brand or a division name.
• Reusing invoice numbers across partial invoices. Every invoice against the PO needs its own unique number; our invoice number generator helps keep the sequence clean.

Our list of common invoice mistakes covers the general errors that apply whether or not a PO is involved. If an invoice is rejected anyway, our guide on handling an invoice dispute explains how to resolve it without souring the relationship.

Do you always need a purchase order?

No. Purchase orders are common with larger companies, public-sector buyers and anyone with formal procurement. Freelancers working with individuals or small businesses often never see one. Even without a PO, a signed quote or contract does a similar job: it records what was agreed so the invoice can be checked against it.

If you do work with PO-driven clients, ask for the PO number before you start work, not when you send the invoice. It saves a round of emails and protects you if the budget is later questioned. Over time, tracking which invoices relate to which PO also makes your accounts receivable easier to manage, and a periodic statement of account helps the client reconcile everything you have billed against each order.

Blanket and standing purchase orders

Not every PO covers a single delivery. Clients who buy from you regularly may issue a blanket purchase order: one PO with a total spending limit and an end date, against which you invoice many times. For example, a blanket PO of $24,000 for monthly IT support from January to December lets you invoice $2,000 each month, quoting the same PO number every time.

With blanket POs, keep a simple running tally on your side: PO value, amount invoiced to date and remaining balance. If the remaining balance will not cover the next invoice, tell the client early so they can raise a new PO before your invoice is due. Showing "PO balance remaining" in your invoice notes is a courtesy many accounts payable teams appreciate.

Need the buyer-side document? Our purchase order template lists the fields suppliers expect, and packing slip vs invoice explains the paperwork that travels with the goods.

Create your PO-matched invoice now

Now that you know the difference between an invoice and a purchase order, bill the right way: open the invoice generator, enter the client's PO number, mirror the PO lines and download a PDF. The Corporate invoice template suits procurement-heavy clients with a clear, formal layout.