What a purchase order actually does
A purchase order does three jobs at once.
It's an offer. When a buyer issues a PO, they're making a formal offer to buy on the terms written on it. In most jurisdictions the PO becomes a binding contract the moment the seller accepts it — by confirming it, by starting the work, or by shipping the goods. That's why the terms on a PO matter: quantities, unit prices, delivery dates and payment terms are the contract, not a wish list.
It's a budget control. Inside a company of any size, the PO is where spending gets approved. Someone with authority signs off before money is committed, not after an invoice lands. This is the whole reason purchase orders exist, and it's why they're common in businesses with more than a handful of staff and rare in businesses with fewer.
It's a reference number. Every document that follows — the delivery note, the invoice, the payment — carries the same PO number. That number is the thread. When a bookkeeper asks "what was this for?", the PO number is the answer, and it's why accounts payable departments reject invoices that don't quote one.
What goes on a purchase order
A purchase order is short. These are the fields that have to be there:
Two things that trip people up. The From and To are reversed compared with an invoice: on a purchase order, the buyer is the one issuing the document, so the buyer's details sit where you'd normally expect the seller's. And the PO number is the buyer's number, not the seller's — the seller quotes it back, they don't invent it.
- PO number — unique, sequential, and quoted on every document that follows.
- Date issued — and, separately, the date the goods or services are needed.
- Buyer details — company name, address, and the person who authorised it.
- Bill To and Ship To — often different addresses. Head office pays; the site takes delivery.
- Seller (vendor) details — the business the order is going to.
- Line items — description, quantity, unit price, line total. Specific enough that the seller can't reasonably send the wrong thing.
- Subtotal, tax and total — the committed amount.
- Delivery date and method — when it's needed and how it's getting there.
- Payment terms — Net 30, Net 15, on delivery, whatever was agreed.
- Authorisation — a name, and on paper POs a signature.
Purchase orders, delivery notes and invoices — the paper trail
Three documents cover the same transaction at three different moments, and they travel in different directions.
The purchase order goes from buyer to seller, before anything happens: please supply this. The delivery note goes from seller to buyer, when the goods arrive: here's what was delivered, please sign. It lists quantities but usually carries no prices at all. The invoice goes from seller to buyer, after the work is done: please pay.
Larger buyers run what's called a three-way match before paying anything: the invoice has to agree with the purchase order and with what was actually received. If the PO says 40 units at $12 and the invoice says 44 units at $12, payment stops until someone explains the difference. This is the single most common reason an invoice sits unpaid for weeks without anyone telling the supplier — it wasn't disputed, it just failed the match.
If you only remember one line: a purchase order is a request to buy, an invoice is a request to be paid. Purchase order vs invoice covers the differences in full.
Types of purchase order you'll come across
Standard PO — one order, specific items, specific date. This is the default and covers most of what you'll see.
Blanket PO (or standing order) — an agreed price and terms across a period, drawn down against as needed. Common where a contractor supplies the same customer repeatedly and nobody wants to raise fresh paperwork every week.
Contract PO — the terms are agreed up front, the items and quantities come later on separate releases.
For a one-person or small business, the one that matters in practice is the blanket PO. If a commercial client gives you a PO number to quote for the next twelve months, that's what it is — and every invoice you send them needs it.
When you don't need a purchase order
Most small businesses never issue one, and that's normal.
If you're a sole trader or a crew of two or three, you buy materials on a card at the supplier's counter. There's no approval step to document, because you are the approval step. Issuing yourself a purchase order adds paperwork and controls nothing.
For service work, a signed estimate or quote does the same job. It sets out scope, price and terms, and once the customer accepts it, you have the agreement a PO would have given you. If you want the protection, tighten the estimate — don't add a second document.
Where purchase orders do land on small businesses is from the other side: commercial and government clients issue them to you. A property manager, a school district or a general contractor will send a PO with a number on it, and their accounts payable system will reject any invoice that doesn't quote that number in the right field. You don't have to create purchase orders to work with them. You have to reference theirs, exactly, every time.
That's worth being blunt about, because a lot of advice on this topic quietly assumes you're the buyer. If you're the one doing the work and sending the bill, you're the seller — and the document you need is an invoice.