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Purchase Orders: What They Are and When You'll Use One

A purchase order is the buyer's own authorization to spend. Here is what it means for you as the seller, how to handle one, and when to issue your own.

Evrenus Fırat Polat · ·

A purchase order (PO) works a little differently from the other documents in an invoicing workflow, because it is typically the client who issues it, not you. Understanding what it represents helps you use it correctly when one shows up.

What a purchase order actually is

A PO is the buyer's internal authorization to make a purchase. It is their own commitment, issued to you as the seller, confirming what they are ordering, at what price, and often referencing an internal budget or cost center. For businesses with a formal procurement process, it is the document that gets approved before any money can move.

A typical PO contains:

  • A unique PO number.
  • Buyer and supplier details.
  • Item descriptions, quantities, and unit prices.
  • Delivery date and delivery address.
  • Payment terms and billing instructions.
  • An authorizing name or signature.

When you'll receive one

Larger or more process-driven clients often issue a PO after accepting your quote or estimate, before work begins. It is their side's paperwork, running parallel to yours. Not every client uses POs. Smaller businesses and individual clients often skip this step and go straight from an accepted quote to work starting.

Government bodies, schools, hospitals, and large companies are the most likely to require one. Ask a new client at the outset: "Do you need a PO number on the invoice?" That one question prevents many payment delays.

What to do with a PO number

Once a client gives you a PO number, reference it on your invoice. Many companies' accounts-payable departments will not process, or even accept, an invoice that lacks a valid PO number, because it is how their system matches the invoice to an already-approved expense. Leaving it off is one of the most common reasons an otherwise correct invoice gets bounced back.

Worked example: checking a PO before you accept

You receive PO-3107 from a client:

  • 3 × on-site workshops at $1,200 = $3,600
  • Travel expenses, capped at $450
  • PO total: $4,050
  • Payment terms: Net 45
  • Delivery window: June 1 to June 30

Before you accept, compare it with your quote. Your quote said $3,600 for the workshops plus actual travel at cost, and Net 30. Two differences: the travel cap and Net 45.

  • If travel will be $520, the extra $70 will not be covered under the cap. Ask for a higher cap now.
  • Net 45 means payment around mid-August for a June 30 invoice instead of late July. That is a 15-day delay on $4,050. Use the invoice due date calculator to see the exact date, and decide whether to accept or ask for a deposit.

Reply in writing: "We accept PO-3107 subject to the travel cap being raised to $550." That keeps the record clean. Any sales tax, VAT, or withholding rules that apply should follow your local requirements. Ask your accountant if the PO does not state them.

Step-by-step: handling a PO

  1. Receive the PO and check the number, date, and signatory.
  2. Compare items, prices, quantities, and terms with your quote.
  3. Raise any differences in writing before starting work.
  4. Confirm acceptance, and optionally issue a sales order.
  5. Deliver, and send a packing slip or delivery note if goods are involved.
  6. Invoice with the PO number, matching the PO values.
  7. Track the PO balance if the order has several deliveries.

If you need to issue one yourself

If your own business buys materials or services from vendors or subcontractors, you can issue POs the same way your clients issue them to you. It formalizes what you are ordering, at what price, before the vendor starts work or ships goods. Our purchase order generator covers this use case directly. It also gives you a clean paper trail for expenses and for checking supplier invoices later.

Purchase order vs. invoice

It is easy to conflate the two since both list items and prices, but they are issued by opposite sides of the transaction at different points in the process. See our guide on purchase order vs. invoice for the distinction. For the seller's counterpart, see sales order vs. purchase order.

Common mistakes

  • Starting work before the PO arrives. If the client's rule is "no PO, no pay," you may wait.
  • Ignoring differences. A PO price that is lower than your quote becomes the price you must bill, unless changed.
  • Losing the PO number. Store it with the job file and on your invoice template.
  • Exceeding the PO value. Ask for a revision first.
  • Invoicing to the wrong address. Many large buyers have a dedicated AP mailbox or portal.

FAQ

Is a PO the same as a contract?

Not exactly. It is an order and may carry terms, but it usually sits alongside a quote, agreement, or master services contract. Check which terms win if they conflict.

Can I refuse a PO?

Yes, if its terms do not match what you agreed. Reply promptly and say what needs to change.

How long is a PO valid?

It depends on the buyer. Look for an expiry or delivery window on the document, and ask if it is missing.

Do I need a PO if the client is a friend or a small business?

Usually not. A written quote acceptance does the same job. Use a PO when the client or your own process asks for one.

A PO is not paperwork for its own sake. For clients who use them, it is the actual internal green light that makes payment possible. Treating a PO number as optional is how invoices end up stuck in someone's queue for reasons that have nothing to do with the work.

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