Purchase Order vs Invoice: Do You Need Both?

What Is a Purchase Order?

‍A Purchase Order (PO) is a document issued by the buyer before work begins. It acts as formal approval to move forward and confirms that the client is authorized to buy from you. In larger companies, this usually means the purchase has already been reviewed internally and approved by the right manager, department, or procurement team.

A PO often includes the buyer’s details, your business details, a PO number, a description of the goods or services being ordered, quantities, agreed prices, and payment terms. In many organizations, it also helps their accounts payable team control spending and track which purchases have been approved.

For you as the seller, a PO matters because it shows the client is not just informally agreeing to the work. They are entering it into their internal system and creating a record that their finance team expects to match against your invoice later.

What Is an Invoice?

‍An invoice is a document issued by you after you have delivered the goods, completed the work, or reached an agreed billing stage. Its purpose is to request payment. While a PO is the client’s authorization to buy, the invoice is your formal request to be paid.

An invoice normally includes your contact details, the client’s details, invoice number, invoice date, payment terms, a list of goods or services provided, the amount due, and any tax information. If the client uses purchase orders, your invoice should also include the PO number.

That reference is often essential. In many corporate systems, the invoice cannot move through the payment workflow unless it matches an existing PO. If the PO number is missing, the invoice may be delayed, rejected, or returned for correction, because naturally humans build systems that collapse over one missing line of text.

How They Work Together

‍In a usual business-to-business workflow, the documents appear in sequence:

• You send a quote, estimate, or proposal
• The client approves the work and issues a purchase order
• You deliver the goods or complete the agreed work
• You issue an invoice that references the PO number
• The client’s accounts payable team matches the invoice against the PO and processes payment

This matching process is one of the main reasons POs exist. The client wants to confirm three things before paying:

• the purchase was approved
• the goods or services were actually delivered
• the invoice amount matches what was authorized

If everything lines up, payment usually moves forward with fewer problems. If it doesn't, the invoice may get stuck while someone chases approvals, corrections, or missing paperwork.

Why the PO Number Is Essential

‍The PO number is the link between the client’s approval and your invoice. Without it, the finance team may have no easy way to confirm that your invoice belongs to an approved purchase.

This is especially common with larger businesses, government bodies, universities, hospitals, and enterprise clients. Their systems are often set up so that invoices without a valid PO number are automatically flagged or rejected. It's not necessarily a dispute about your work. It is often just a process issue. Annoying, yes. Predictable, also yes.

Adding the PO number to your invoice helps:

• reduce payment delays
• avoid invoice rejections
• make it easier for AP to process your payment
• create a stronger audit trail for both sides

If the client gave you a PO, use it exactly as provided and place it somewhere easy to find on the invoice.

Do You Always Need Both?

‍No. Not every client uses purchase orders.

Small businesses, sole traders, and individual clients often skip POs completely. In those cases, the agreement might be handled through an estimate, proposal, contract, email approval, or even a simple written confirmation. You still send an invoice when it is time to get paid, but there may be no PO involved.

Purchase orders are most common when the client has:

• a procurement department
• a formal accounts payable process
• internal spending controls
• multiple people approving purchases
• strict documentation requirements

If your client is a larger organization, assume a PO may be required and ask early. It is much easier to check before starting than to discover after delivery that payment cannot be processed.

What Happens If You Invoice Without a PO?

‍If the client requires a PO and you send an invoice without one, several things can happen:

• the invoice may be rejected outright
• the invoice may sit unpaid while the client requests a PO retroactively
• payment may be delayed for weeks while internal approvals are sorted out
• the client may ask you to resubmit the invoice with the correct reference

This can be especially frustrating if the work is already done and everyone agrees you should be paid. The issue is not the value of the invoice. It is that the paperwork does not fit the client’s internal process.

That is why it is smart to confirm billing requirements before the work starts, especially with new B2B clients.

What to Do When No PO Exists

‍If a client says, “Just send the invoice,” but you suspect they normally use purchase orders, pause and check. A simple message can save a lot of delay:

“Could you send me the PO number to reference on the invoice? I want to make sure it goes through your system smoothly.”

If they confirm that no PO is needed, keep that confirmation in writing. An email is usually enough. If they say a PO will be issued later, it is often better to wait for it before starting work or at least before invoicing.

If the client genuinely does not use POs, you can proceed with the invoice as normal. The main thing is not to guess. Guessing is a lovely way to create unpaid invoices and email chains no one wants.

PO vs Invoice at a Glance

• Purchase Order: Issued by the buyer before work starts to authorize the purchase
• Invoice: Issued by the seller after work is completed or billed to request payment
• PO number: Used by the client to approve and track spending
• Invoice number: Used by you to track billing and payment

They're elated, but they do different jobs. One authorizes the purchase. The other asks to be paid for it.

Final Take

‍Use a purchase order as the client’s approval to buy and an invoice as your request to be paid. For corporate and government clients, always reference the PO number on your invoice. For smaller clients, a PO is often unnecessary, but it is always worth confirming before work begins.