Need the whole process rather than the definition? See our guide to purchase orders and invoice approval — getting a PO number, three-way matching, and why invoices fail approval.
Purchase Order vs Invoice Two essential documents in the B2B payment cycle — here's what each one does
Quick Definitions
Purchase Order (PO)
A document the BUYER sends to the SELLER to authorize a purchase. It comes before the work starts.
Created by: Buyer
Issued: Before work begins
Invoice
A document the SELLER sends to the BUYER to request payment. It comes after the work is done.
Created by: Seller
Issued: After work is complete
The B2B Payment Flow
Buyer sends PO
Authorises the purchase
Seller confirms
Acknowledges the PO
Seller delivers
Goods or services provided
Seller sends Invoice
References the PO number
Buyer pays
Payment settled
Key Differences
| Purchase Order | Invoice | |
|---|---|---|
| Who creates it | The buyer | The seller |
| When is it issued | Before work starts | After work is complete |
| Purpose | Authorise a purchase | Request payment |
| Contains | Description, quantity, agreed price, delivery terms | Itemised charges, taxes, total due, payment terms |
| Legally binding? | Yes, once seller accepts | Yes — creates payment obligation |
When You Don't Need a PO
- Most freelance or consumer work — individuals and small businesses rarely use formal POs
- One-off projects with small businesses where a verbal or email agreement is sufficient
- When you invoice individuals (consumers) rather than businesses
When POs Matter
- Corporate or enterprise clients with formal procurement and accounts payable departments
- Government contracts — POs are almost always required by public sector buyers
- Any client who asks you to include a PO number on your invoice
Frequently Asked Questions
Do I need a purchase order to invoice someone?
No. Most freelancers and small businesses invoice without a PO. POs are mainly used by larger companies with formal procurement processes. If a client gives you a PO number, just reference it on your invoice.
What should I put on an invoice when a client has a PO?
Include the PO number prominently on your invoice, usually near the invoice number or in a dedicated "PO Number" field. This ensures the client's accounts payable team can match your invoice to their records and process it without delay.
Is a purchase order a contract?
A PO becomes a binding contract when the seller accepts it. It specifies the agreed price, quantity, and delivery terms. Treat it seriously — deviating from the agreed scope without a change order can create disputes.
What's the difference between a PO and a quote?
A quote comes from the seller showing what they'll charge. A PO comes from the buyer confirming they want to proceed at that price. Quote → PO → Invoice is the full B2B cycle for formal procurement.
Can I create a purchase order in Zap Bill?
Zap Bill focuses on invoicing and quoting for the seller side. If you need to issue POs as a buyer, a simple template in your word processor works well for most small businesses. For invoicing, Zap Bill handles everything you need.
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