Purchase Orders vs. Invoices: What’s the Difference?

What is a purchase order (PO)?

A purchase order is the first step in a transaction. It is a formal document sent from the buyer to the seller, listing exactly what the buyer wants to purchase.

Think of it as a "contract in waiting." After receiving a PO, the seller confirms they can fulfil the order. POs are crucial for tracking inventory, managing budgets, and ensuring that both parties agree on the price and quantity before any money changes hands.

What information should be on a PO?

To ensure your purchase orders are effective and professional, they should include:

• A unique PO number: Essential for tracking the order through your system.

• Date of issue: When the request was made.

• Buyer and seller details: Names, addresses, and contact information for both parties.

• Itemised list: Detailed descriptions of products or services, quantities, and unit prices.

• Delivery details: The shipping address and the expected delivery date.

• Payment terms: How and when you intend to pay (e.g., "Net 30 days").

Is a purchase order a legally binding contract?

Yes. A purchase order becomes a legally binding contract the moment the seller formally accepts it (either by signing it, sending a confirmation, or beginning to fulfil the order). This protects both parties: the seller is obligated to deliver the goods, and the buyer is obligated to pay the agreed price.

What is an invoice?

An invoice is a document sent by the seller to the buyer after the goods or services have been provided. It serves as a formal request for payment.

While a PO starts the transaction, the invoice concludes it. It confirms that the terms agreed upon in the PO have been met and details how much the buyer now owes. For VAT-registered businesses in the UK, invoices are also a legal requirement for tax reporting.

Essential information required on every invoice

To be valid and professional, every invoice must include:

• "Invoice": The word must be clearly visible.

• Unique invoice number: A sequential number for your records.

• PO number: The corresponding purchase order number (if one was issued).

• Business information: Your name, address, and contact details.

• Customer information: The name and address of the customer.

• Supply date: When the goods or services were provided.

• Total amount due: Including a breakdown of individual items and VAT (if applicable).

• Payment terms: When and how you expect to be paid (e.g., "Net 30").

• VAT registration number: Your unique 9-digit number (required if your business is VAT registered).

• VAT breakdown: The specific VAT rate applied and the total amount of VAT charged.

The workflow: from order to payment

For most businesses, the order-to-payment cycle follows a standard flow:

1. Buyer PO: The buyer identifies a need and sends a PO to the supplier.

2. Acceptance: The supplier confirms they can meet the request.

3. Delivery: The supplier ships the goods or performs the service.

4. Invoice: The supplier sends the invoice to request payment.

5. Payment: The buyer pays the supplier based on the invoice terms.

The 3-way match: PO, delivery note, and invoice

Large companies and efficient small businesses use a process called 3-way matching before paying any bill. This prevents fraud and errors by cross-referencing three documents:

1. The purchase order: Did we actually order this?

2. The delivery note (or goods received note): Did we actually receive this?

3. The invoice: Is the supplier charging us the correct amount?

If all three match, the payment is authorised. If they don't, it's a red flag that requires investigation before any cash leaves the business.

Key differences: purchase order vs. invoice

While they relate to the same transaction, their roles are distinct:

The bottom line

Purchase orders and invoices are the bookends of a successful business transaction. The PO protects you by locking in prices and quantities, while the invoice provides the formal record needed for your tax and accounting.

Using both documents correctly doesn't just prevent errors—it builds a professional reputation with your suppliers. To make the process even smoother, you can pay your finalised invoices with a Capital on Tap Business Credit Card. This allows you to track all your spending in one place and sync your data directly with accounting software like Sage, Xero or QuickBooks. Apply now and get a decision in minutes.

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Frequently asked questions

This guide does not constitute financial advice. Please consult an accountant or financial advisor if you would like more information.