What is Purchase Order?

A purchase order (PO) is a formal document issued by a buyer to a seller, authorizing the purchase of specific goods or services at specified prices and quantities. When a buyer issues a purchase order and a seller accepts it, the PO becomes a legally binding contract. Unlike an invoice (which the seller issues to request payment), a purchase order is issued by the buyer to specify and authorize what they want to buy. For freelancers and small business owners, purchase orders most commonly appear when clients are large organizations with formal procurement processes -- corporations, government agencies, universities, and healthcare systems. These organizations require a PO number before approving any vendor invoice, ensuring all spending is pre-authorized. Understanding purchase orders helps you work smoothly with corporate clients, invoice correctly (including the PO number), and ensure your payment is processed without delay.

The purchase order process works as follows: the buyer identifies a need, creates a purchase order specifying the item or service, quantity, price, and vendor, and routes the PO through internal approval. Once approved, the PO is sent to the vendor. The vendor reviews the PO, confirms they can fulfill the order as specified, and formally accepts it. Work proceeds. Upon completion, the vendor (you, as the freelancer) issues an invoice that references the PO number. The buyer's accounts payable team matches the invoice to the PO (and often to a goods receipt), verifies the quantities and prices match, and schedules payment. This three-way match -- PO, invoice, goods receipt -- is the standard control process in procurement-heavy organizations. Without the PO number on your invoice, payment may be delayed while the client's team hunts for the authorization document.

Many freelancers who work primarily with small business clients rarely encounter purchase orders. But those who target corporate, government, or institutional clients will find POs are mandatory. Best practice when a client mentions a PO: ask for the PO number before issuing your invoice, and include it prominently on the invoice. Never start work on a PO-required contract until you have the PO number in hand -- 'the PO is coming' is not authorization. Some clients will issue the PO before work begins; others issue it after project completion, which creates a timing gap between work delivered and payment authorization. Clarifying the client's PO process before starting work prevents invoicing delays. For very large PO-driven contracts, it is reasonable to require the PO as a condition of commencing work, alongside a signed contract.

A purchase order is issued by the buyer to authorize and specify a purchase. An invoice is issued by the seller to request payment for completed work. They are opposite-direction documents in the same transaction. The PO comes first (buyer to seller), establishing what is being bought and at what price. The invoice comes after work is completed (seller to buyer), requesting payment for the authorized work. For the invoice to be paid efficiently, it must reference the PO number and match the PO's specified items and prices. Discrepancies between the PO and the invoice -- different quantities, different prices, additional items not in the PO -- will delay payment while the client investigates. Keeping your invoice tightly aligned with the PO terms is essential for timely payment.

Step 1: When discussing a new project with a corporate or institutional client, ask early in the conversation whether a PO will be required. Step 2: Before starting work, request the PO document and verify that it accurately reflects the agreed scope and price. Step 3: Review the PO payment terms -- large organizations often have standard terms (Net 45, Net 60) that you should be aware of before accepting the engagement. Step 4: When the project is complete, create your invoice referencing the PO number prominently. Match your invoice items and prices exactly to the PO. Step 5: Submit the invoice per the client's instructions -- some organizations require invoices through an online vendor portal, not by email. Step 6: Follow up according to the client's payment cycle if payment has not arrived by the due date.

Eonebill makes it easy to include purchase order numbers and references on invoices, ensuring your invoices are immediately processable by clients with formal procurement requirements. A clearly formatted invoice with the PO number prominently displayed reduces the risk of payment delays caused by missing reference information. The [free invoice generator](/free-tools/invoice-generator) allows you to add custom fields including PO numbers, vendor IDs, and other client-specific reference information. [Eonebill pricing](/pricing) plans support the professional invoicing infrastructure that corporate and institutional clients expect, giving you the tools to work seamlessly with large organizations that require formal documentation and reference matching.

1. Starting work without a PO when the client's procurement process requires one: 'we will get you the PO soon' is not authorization; wait for the actual document. 2. Invoicing without the PO number when one was issued: the PO number is the link that connects your invoice to the client's approval; without it, your invoice may sit in a queue indefinitely. 3. Invoicing for amounts or items not in the PO: any discrepancy triggers a matching exception that delays payment; discuss changes before invoicing. 4. Ignoring the PO payment terms: large organizations often have longer payment terms (Net 45-60) baked into their POs; factor these into your cash flow planning. 5. Submitting invoices through the wrong channel: many organizations require invoices through specific portals or email addresses; ask the correct submission process upfront.

[Purchase Invoice](/glossary/purchase-invoice) -- the vendor invoice that corresponds to an accepted purchase order. [Define Invoice](/glossary/define-invoice) -- the seller's counterpart to the buyer's purchase order. [What Is a Contract](/glossary/what-is-a-contract) -- an accepted purchase order functions as a contract. [Payment Application](/glossary/payment-application) -- the process of matching PO invoices to payments received.