What Is a Purchase Order?
A purchase order (PO) is a document the buyer sends the seller. It's the buyer's official commitment: "we agree to buy this, at this price, under these terms." If you sell to companies, POs run the show.
The buyer's side of the paperwork
Most invoicing documents come from the seller. The purchase order is the exception — it comes from the buyer. When a company's employee wants to buy something, they usually can't just agree verbally. Their finance department requires a PO: a numbered document that authorizes the purchase, states exactly what's being bought, and caps the amount.
The PO protects the buyer. It proves the purchase was approved internally, locks in the agreed price, and gives their accounts team a reference to match against your invoice. When your invoice arrives, the first thing their system does is check: does this match an approved PO?
For freelancers, the practical meaning is simple: no PO, no payment. Many companies — especially larger ones, universities, and government agencies — literally cannot pay an invoice that doesn't reference a valid PO number. The money is there; the process just won't release it.
What a purchase order contains
A typical PO includes the buyer's company details, the seller's details, a unique PO number, a description of the goods or services, quantities and agreed prices, the delivery date or schedule, and the billing address where the invoice should go.
That last item matters more than you'd think. Big companies often have a dedicated invoicing portal or AP email address, and sending your invoice to the person you worked with — instead of the address on the PO — is a classic cause of month-long delays.
The PO number is the key that links everything. Quote it on your invoice and the buyer's system can match the three documents — PO, invoice, and delivery confirmation — automatically. Miss it and your invoice lands in a manual-review pile.
Invoice with the PO number, free
Billia's invoice generator has a field for PO numbers, so your invoices match your clients' systems the first time.
Create an invoice — freeHow POs fit into your workflow
- Ask early. Before starting work, ask: "Do you need a PO for this?" If yes, wait for the PO number before you begin — or at least before you invoice.
- Quote against the PO scope. Your quote and the buyer's PO should describe the same work at the same price. If they differ, resolve it before work starts, not at billing time.
- Put the PO number on the invoice. Prominently — near the invoice number or in the notes. This is the single highest-ROI line on a B2B invoice.
- Invoice to the right address. Use the billing details from the PO, not your contact's email, unless told otherwise.
- Don't exceed the PO amount. If the work grows beyond the PO's cap, get an amended PO first. Invoicing above the approved amount is the fastest route to a rejected invoice.
Treat the PO as part of the job's paperwork from day one, not an afterthought at billing time. A freelancer who asks about the PO upfront looks professional; one who asks after invoicing looks disorganized.
POs vs. the documents you send
It's easy to confuse a PO with a quote or contract, so here's the clean split: the quote is your proposed price (you send it), the purchase order is the buyer's acceptance and authorization (they send it), the contract — if there is one — is the mutual agreement covering terms, and the invoice is your bill (you send it).
In smaller jobs the PO and the contract collapse into one: the PO is the agreement. In larger ones, the PO references the contract. Either way, your invoice references the PO number, and the chain is complete: quote → PO → work → invoice → payment.
If a client sends you a PO with terms you can't meet — a 90-day payment term, say, or a liability clause — push back before accepting it. An accepted PO is a commitment, just like an accepted quote.
Invoice with the PO number, free
Billia's invoice generator has a field for PO numbers, so your invoices match your clients' systems the first time.
Create an invoice — freeA purchase order is the buyer saying "yes" in writing — with a number, a price cap, and an approval trail behind it. Ask for it early, quote against it, put its number on your invoice, and send that invoice where the PO says to send it. Do that and corporate clients pay you like clockwork.
Create an invoice with the PO number included — free, no signup.
Frequently asked questions
Do I need a purchase order for every client?
No — POs are mainly a corporate thing. Individual clients and small businesses rarely use them. But whenever a client mentions one, treat it as mandatory: ask for the PO number before you start work, and reference it on your invoice.
What if the client won't give me a PO number?
Get the agreement in writing another way — a confirmed email stating the scope, price, and who approved it. A PO is the buyer's internal process; if they don't use POs, a written confirmation from someone with authority serves the same protective purpose for you.
Where do I put the PO number on an invoice?
Near the top, close to your invoice number — or in a dedicated 'PO number' field if your invoicing tool has one. Billia's generator includes a PO field for exactly this. The goal is that the client's accounts team spots it instantly.
Can I start work before the PO arrives?
You can, but you take on risk: if the PO never comes or comes back with different terms, you've worked without an approved commitment. For new corporate clients, waiting for the PO is the safer move. For trusted repeat clients, many freelancers start on a confirmed email and slot the PO number in at billing time.