People often use purchase order and invoice as if they were interchangeable. They are not. A purchase order starts the buying process from the client side. An invoice finishes it from the supplier side.
If you understand that sequence, you avoid two common problems: starting work before the client has properly approved it, and sending an invoice that gets rejected because no PO number exists.
What is a purchase order?
A purchase order or PO is a document issued by the buyer. It confirms that they want to buy specified goods or services under agreed terms.
A purchase order usually includes:
- buyer details,
- supplier details,
- a PO number,
- description of the goods or services,
- quantities or scope,
- agreed pricing,
- approval information.
For larger organisations, the PO is often required before the supplier starts work.
What is an invoice?
An invoice is issued by the seller after goods are delivered or services are performed. It requests payment and becomes part of the accounting record.
So in practical terms:
- the purchase order says, "Please provide this."
- the invoice says, "This has been provided. Please pay this amount."
Which comes first?
In a standard workflow, the order is:
- Quote or proposal
- Purchase order from the client
- Delivery of goods or services
- Invoice from the supplier
- Payment
Not every small business uses formal POs, but many mid-sized and enterprise clients do.
Purchase order vs invoice: side-by-side
| Document | Issued by | Timing | Main purpose | | -------------- | --------- | --------------- | ----------------------------------- | | Purchase order | Buyer | Before delivery | Approve and authorise the purchase | | Invoice | Seller | After delivery | Request payment and record the sale |
Why do finance teams ask for a PO number?
Because their accounts-payable system often will not release payment without matching the invoice to an approved purchase order.
If your client uses that workflow, including the PO number on the invoice can make the difference between getting paid on time and waiting for someone internal to fix the paperwork.
Can freelancers work without a purchase order?
Yes. Many freelancers work on the basis of:
- an accepted quote,
- a signed contract,
- or written email approval.
That is common with startups, small agencies, and direct clients.
But if the client has a procurement process, ask this early: "Do you need me to reference a PO number on the invoice?" It is a small question that prevents avoidable payment delays.
What if there is no purchase order?
That does not mean you cannot invoice. It simply means another document is carrying the commercial approval, such as a quote or contract. Our guide on invoice vs quote covers that relationship in more detail.
Common mistakes
Starting work before internal approval exists
The hiring manager may say yes, but finance may still need a PO.
Sending an invoice with no reference to the PO
If the client issued PO-44812, put it on the invoice. Otherwise, AP may not know where to code the bill.
Confusing a purchase order with a proforma invoice
A purchase order is created by the buyer. A proforma invoice is created by the seller. They may appear in the same workflow, but they are not the same document.
Summary
The difference is straightforward: the purchase order approves the spend, while the invoice requests payment after delivery. When you know which document the client needs and when, payment friction drops sharply.
Invoice Creator helps you generate a clear, professional invoice in minutes and export it as a PDF. If you want a ready-made starting point, use the site’s invoice templates or the dedicated freelancer invoice page.