What Information Should an Invoice Include?

Purchase orders and invoices often appear in the same transaction, but they are not the same document. A purchase order usually comes from the buyer before the purchase is completed. An invoice usually comes from the seller and requests payment for the goods or services supplied.

Understanding the difference is especially important when you start working with larger companies. Many organizations will not process an invoice unless it includes a valid purchase order number.

What is a purchase order?

A purchase order, often called a PO, is a document a buyer sends to a supplier to confirm what they want to purchase. It can include products or services, quantities, agreed prices, delivery details, payment terms, and a unique PO number.

The buyer creates the purchase order. It acts as an internal and external record of what has been authorized.

What is an invoice?

An invoice is normally created by the seller after the agreed billing event. It asks the customer to pay a specific amount for products or services.

A professional invoice normally includes seller and customer details, invoice number, invoice date, due date, description of what was supplied, amounts, tax where applicable, and the total due. See our guide to what information an invoice should include.

Purchase order vs invoice at a glance

Feature	Purchase Order	Invoice
Created by	Buyer	Seller
Main purpose	Authorize a purchase	Request payment
Typical timing	Before supply	After supply or agreed billing point
Main reference	PO number	Invoice number

Example of how they work together

Imagine a company wants to buy £2,000 of design services from your business. After agreeing the scope and price, the customer raises purchase order PO-58321 and sends it to you.

You complete the work and create Invoice INV-1048 for £2,000. The invoice includes PO-58321 in the reference field. The customer’s accounts team matches your invoice to the purchase order and, if the details agree, moves it through the payment process.

Why purchase order numbers matter

For a small client, an invoice may be approved directly by the owner. Larger companies often have separate departments for procurement, operations, and accounts payable. The PO connects the original authorization to the later invoice.

If the PO number is missing, accounts payable may not know which manager approved the expense. The invoice can be rejected or placed on hold while someone investigates.

Does every invoice need a purchase order?

No. Many transactions do not use purchase orders. Freelancers, small businesses, retail sales, direct consumer work, and simple service arrangements may rely on a quotation, contract, or email approval instead.

You only need a PO reference when the customer’s process requires one or when it is part of your agreement.

Is a quotation the same as a purchase order?

No. A quotation is normally created by the seller to propose prices and terms before the customer buys. A purchase order is created by the buyer to authorize the purchase.

1. Seller sends quotation.
2. Buyer accepts and raises purchase order.
3. Seller supplies goods or services.
4. Seller sends invoice.
5. Buyer matches invoice to PO and pays.

Read invoice vs quotation for more detail on those two documents.

Can a purchase order replace an invoice?

Usually no. A purchase order confirms the buyer’s intention and authorization to purchase. It is not normally the seller’s request for payment.

The seller still issues an invoice so the customer knows the exact amount due, payment date, tax treatment, and payment instructions.

What should you do when a client asks for a PO?

Ask them to provide the purchase order before you invoice. Keep the PO number with your project records. When you prepare the invoice, copy the number exactly into the PO or reference field.

Also check whether the customer requires a specific legal entity name, billing address, department, tax number, supplier number, or portal upload.

What if the PO amount is wrong?

Do not simply issue a higher invoice and assume the customer will pay it. If the scope changed or additional work was approved, ask the client to update the purchase order or provide the authorization required by their process.

Otherwise the invoice may fail the matching process.

Should the PO number appear on the quotation?

A purchase order often does not exist when the quotation is first sent. If the customer creates a PO after accepting the quote, include the PO number on later documents such as the invoice.

You can create the pricing document using the Quotation Generator.

Create an invoice with a PO reference

Add your own invoice number, customer details, items, taxes, totals, due date, and the customer’s PO number.

Create an Invoice

Common PO and invoice mistakes

• Using the PO number as your invoice number.
• Sending an invoice before the required PO exists.
• Entering the wrong PO number.
• Billing above the authorized value without approval.
• Ignoring customer-specific billing instructions.

Frequently asked questions

Who sends the purchase order?

The buyer or customer normally creates and sends the PO.

Who sends the invoice?

The seller or supplier normally sends the invoice to request payment.

Can an invoice be issued without a PO?

Yes, unless the customer requires a PO as part of its purchasing process.

Final thoughts

A purchase order authorizes a purchase from the buyer’s side, while an invoice requests payment from the seller’s side. When both documents are used, accurate references help the transaction move smoothly. If a customer requires a PO, get the number early and include it exactly on the invoice.