If you have ever worked with a corporate client or government agency, you have probably been asked to "quote a PO number" on your invoice. But what exactly is a purchase order, how is it different from an invoice, and why does it matter for getting paid?

This guide clearly explains both documents, how they work together, and when you need each one.

What Is a Purchase Order (PO)?

A purchase order (PO) is a document created and sent by the buyer to the seller before any work begins. It is the buyer's official, legally binding offer to purchase specific goods or services at an agreed price.

Think of a PO as the buyer saying: "We officially authorise you to do this work for this price."

A purchase order typically includes:

A unique PO number (e.g., PO-2026-00892)
The buyer's company name and billing address
The seller's name and address
A list of goods or services ordered with quantities and agreed prices
The delivery address or service location
The delivery or completion date
The buyer's payment terms
An authorised signature from the buyer

What Is an Invoice?

An invoice is a document created and sent by the seller to the buyer after the work is completed or goods are delivered. It is a formal request for payment.

Think of an invoice as the seller saying: "We have completed the agreed work — here is what you owe us."

An invoice typically includes:

A unique invoice number
The seller's details (name, address, tax ID)
The buyer's details
The PO number from the buyer (if applicable)
An itemised list of goods or services delivered with quantities and prices
The total amount due including taxes
The due date and payment instructions

Purchase Order vs Invoice: Key Differences

Feature	Purchase Order (PO)	Invoice
Created by	Buyer	Seller
Timing	Before work begins	After work is completed
Purpose	Authorises a purchase	Requests payment
Legal status	Offer to buy — binding when accepted	Payment demand — binding obligation
Contains price	Agreed price (from quote)	Actual price charged
Who uses it	Businesses, government, large organisations	All businesses and freelancers

How POs and Invoices Work Together

In a standard business transaction, the two documents work in sequence:

Seller sends a quote with their pricing
Buyer approves and issues a PO referencing the quote
Seller completes the work or delivers goods
Seller sends an invoice referencing the PO number
Buyer's accounts payable team matches the invoice to the PO (called "3-way matching")
Payment is processed once the match is confirmed

This is why corporate clients always ask you to include the PO number on your invoice — without it, their accounts payable system cannot match the invoice and your payment will be delayed or rejected.

Do Freelancers Need Purchase Orders?

Freelancers working with small businesses or individual clients often do not deal with POs — they simply send a quote and then an invoice. However, if you work with:

Large corporations
Government agencies
Hospitals, universities, or councils
Agencies billing on behalf of clients

…you will almost certainly be asked for a PO number before work starts. Never start work for a large organisation without getting a PO first. Without a PO, there is no guarantee your invoice will be paid — even after completing the work.

When to Use Each Document

Use a Purchase Order when:

You are buying goods or services and need formal internal approval
You need to commit to specific quantities, prices, and delivery dates
Your company requires audit trails for spend (common for businesses over $1M revenue)
You are working with a supplier who requires PO confirmation before starting

Use an Invoice when:

You have completed work or delivered goods
You need to formally request payment
You are a freelancer or sole trader billing a client
You need a record of the transaction for tax purposes

Create a Professional Invoice in Seconds

Whether you are responding to a PO or billing an individual client, our free invoice generator makes it easy to create a professional, PDF-ready invoice in under 60 seconds. Include your PO reference number, itemise your services, and download instantly.

→ Create a Free Invoice Now

Need to send a quote before a client issues a PO? Use our free quote generator to send a professional price quotation in seconds.

Frequently Asked Questions

Can an invoice replace a purchase order?

No. An invoice and a purchase order serve opposite purposes. A PO comes from the buyer before work starts; an invoice comes from the seller after work is done. You cannot use an invoice as a substitute for a PO because an invoice does not authorise a purchase — it requests payment for work already authorised.

What happens if I invoice without a PO number?

For large organisations, invoices without a PO number are often returned or rejected by accounts payable, causing significant payment delays. Always get a PO number from corporate clients before starting work and include it prominently on your invoice.

Is a purchase order legally binding?

Yes. Once you (the seller) accept a purchase order — either by signing it, sending a written acknowledgement, or beginning the work — it becomes a legally binding contract. Both parties are obligated to fulfil the agreed terms: the seller to deliver the goods/services, and the buyer to pay the agreed price by the agreed date.

Do I need to use accounting software to manage POs and invoices?

Not necessarily. For small businesses and freelancers, a simple invoice generator is sufficient for managing invoices. For businesses that issue many POs, dedicated accounting software (like Xero or QuickBooks) can automate the PO-to-invoice matching process. But for getting started, a free invoice tool is all you need.