Purchase Order vs Invoice: Who Sends Which, and When

A purchase order vs invoice comparison comes down to direction and timing: the buyer sends the purchase order before the goods or work, to say what they want at what price; the seller sends the invoice after, to ask for payment. They list the same items and are easy to confuse. This site makes both, which is why the table below ends with the one turning into the other.

Which one do you need?

If you are	Send
The buyer, ordering goods or work at agreed prices	A purchase order	Purchase order generator
The seller, asking to be paid for what you delivered	An invoice, quoting the PO number if there was one	Invoice builder
The seller, pricing the job before any order exists	A quote or estimate	Quote generator

How the purchase order becomes the invoice

1. The buyer issues a PO: items, quantities, unit prices, delivery date, ship-to address, payment terms, and a PO number. Once the vendor accepts, the price is fixed.
2. The vendor delivers, and the buyer records what arrived.
3. The vendor invoices the same lines at the same prices, with the PO number in the reference field, and adds an invoice number and a due date from the agreed terms.
4. The buyer checks the invoice against the PO and the delivery, then pays by the due date.

The purchase order generator and the invoice builder share one sheet, so the same line items serve both documents; the invoice's PO or reference field carries the number across. Tipalti and Order.co describe the same sequence from the accounts payable side.

The three-way match, in one paragraph

Before paying, the buyer confirms three documents agree: the purchase order (what was ordered), the receiving record (what arrived), and the invoice (what was billed). If the quantities and prices line up, the invoice is approved; if not, it is queried. A one-person business does this by eye; larger buyers automate it, which is why an invoice that quotes the PO number and matches it exactly gets paid faster. Spendflo covers the automated version.

When there is no purchase order

Most small purchases never have one. Freelance work, small repairs, subscriptions, and anything urgent usually go from a quote or a conversation straight to an invoice. A PO is worth issuing, or asking for, when the amount is large enough that both sides want the price and scope fixed in writing before the work starts, or when the buyer's finance team requires one to approve payment. If a client tells you "we need a PO for that," send them a quote first; their PO will come back with a number to put on your invoice.

What each document contains

Both carry both parties' details, a date, itemized lines with quantities and prices, and totals. The purchase order adds a PO number, a delivery date, and a ship-to address; the invoice adds an invoice number, a due date, payment instructions, and the PO number as a reference. Adobe lists the same split. The guide to what is an invoice covers the invoice side field by field.

Which document do I need?

• Invoice vs receiptAsking for payment, or proving it.
• Invoice vs quote vs proformaBefore the work, after agreement, or after delivery.
• Invoice vs bill vs statementOne transaction, or the whole account.
• Purchase order vs invoiceThe buyer's order, or the seller's bill.

This comparison was written by Ready Utilities. The distinctions were checked against Tipalti, Order.co, Spendflo, and Adobe, and the sequence matches how the site's own purchase order generator and invoice builder share a sheet. It is reviewed when those sources update their guidance.

Frequently asked questions

Can a purchase order be used as an invoice?

No. A purchase order is the buyer's request and authorization; an invoice is the seller's request for payment. They travel in opposite directions and one cannot stand in for the other, even though they list the same items.

Do I need a purchase order for every purchase?

No. Businesses use POs for planned or larger purchases where they want approval before money is committed. Small, recurring, or urgent costs often go straight to an invoice.

Which comes first, the PO or the invoice?

The purchase order. The buyer issues it, the vendor fills the order, and then the vendor invoices against the PO number. Quoting that number on the invoice is how the buyer's accounts payable matches the two.

Is a purchase order legally binding?

Once the vendor accepts it, yes: it is an offer to buy at stated prices that the vendor has agreed to fill. An invoice on its own is not a contract; it is a record of what is owed under the agreement that already exists.

What is a three-way match?

The buyer's check that what was ordered (the PO), what arrived (the receiving record or delivery note), and what was billed (the invoice) all agree before the invoice is paid. Small businesses do it by eye; larger ones automate it.

What if the invoice does not match the PO?

The buyer queries the difference before paying: quantity, price, or an item that was not ordered. As the vendor, invoicing exactly what the PO said, with the PO number on the invoice, avoids the hold.