If a business client has ever sent you a "PO number" and asked you to reference it on your invoice, you have already encountered a purchase order. Understanding the difference between an invoice and a purchase order — and why they are used together — helps you work more professionally with corporate clients and get paid without delays. Get this wrong and your invoice may sit unprocessed in a client's accounts payable queue for weeks, waiting to be matched against a PO that nobody told you about.
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What Is a Purchase Order (PO)?
A purchase order is a document sent by the buyer to the seller, before work begins or goods are delivered. It is the buyer's official commitment to pay for specified goods or services at an agreed price.
Purchase orders are created by the buyer's procurement or finance department. They serve several purposes for the buying organisation:
- Spend authorisation: The PO is internal approval for a purchase — it shows that someone with authority has approved the expense before money is committed.
- Budget tracking: Finance teams use POs to track commitments against department budgets in real time.
- Supplier communication: The PO tells the supplier exactly what is being ordered, at what quantity, at what agreed price, and any delivery requirements.
- Three-way matching: When your invoice arrives, the client's accounts payable team matches it against the PO and (for physical goods) the goods receipt note. All three must match before payment is released.
A purchase order typically contains: a unique PO number, a description of goods or services ordered, quantities, agreed prices, the buyer's name and address, the supplier's name, and any delivery or completion date requirements.
What Is an Invoice?
An invoice is a document sent by the seller to the buyer, requesting payment after work is completed or goods are delivered. As a freelancer or contractor, the invoice is the document you create.
A complete invoice includes: your business name and contact details, the client's name and address, a unique invoice number, the invoice date and payment due date, an itemized list of services or goods provided with quantities and rates, the total amount due, your payment terms, and your accepted payment methods.
Unlike a purchase order — which is a commitment to buy — an invoice is a demand for payment. It triggers the payment process on the client's end.
Purchase Order vs Invoice — Side by Side
| Feature | Purchase Order | Invoice |
|---|---|---|
| Who creates it | Buyer (your client) | Seller (you) |
| When issued | Before work starts | After work is completed |
| Purpose | Commit to buying / approve spending | Request payment |
| Contains a price | Yes (agreed price) | Yes (actual price charged) |
| Legal status | Offer to buy / spending authorisation | Payment demand |
| Triggers payment? | No — it authorises the work | Yes — it requests the money |
| Who uses it | Corporate buyers, procurement teams | Freelancers, businesses, suppliers |
| Required for freelancers? | Only if the client requests it | Always — for every job |
How Purchase Orders and Invoices Work Together
For freelancers and contractors working with corporate clients, POs and invoices operate in a defined sequence. Here is the complete workflow:
- 1Client sends you a Purchase OrderThe client's procurement team issues a PO before work begins: "We authorise 20 hours of design work at $80/hr — PO Number: PO-2026-047." This is your green light to start.
- 2You complete the agreed workDo the work as specified in the PO. If scope changes, request an amended PO or a new PO for the additional work before proceeding.
- 3You issue your invoice referencing the PO numberCreate your invoice and include the PO number prominently — in the reference field or in the Notes section: "Invoice for design services per PO-2026-047."
- 4Client's accounts payable team matches the invoiceThey compare your invoice against the original PO in their system. If the amounts, services, and PO number match, the invoice is approved for payment.
- 5Payment is processed and releasedOnce matched and approved, your invoice enters the payment run. The speed depends on their payment terms (Net 30, Net 60, etc.) and payment processing schedule.
Always ask new corporate clients if they use purchase orders. If they do, ask for the PO before starting work. Your invoice may be held up indefinitely in their payment system until it matches a valid PO number — even if the work was excellent and the client is perfectly happy.
Do Freelancers Need to Issue Purchase Orders?
No. Purchase orders are created by buyers, not sellers. You never create a PO for your client — they create one for you, if their procurement process requires it.
As a freelancer or contractor, your role in this process is straightforward:
- You receive purchase orders from clients who use them.
- You create and issue invoices to request payment.
- You do not create purchase orders for your own work.
The one exception: if you hire subcontractors yourself to help deliver a project, you might issue purchase orders to them to authorise and document what they are being hired to do. But this is only relevant if you are running a small agency or team, not as a solo freelancer.
Do You Need a PO Number on Your Invoice?
It depends entirely on whether your client uses a purchase order system:
- Corporate and enterprise clients almost always have formal procurement processes and will require a PO number on every invoice. Without it, their accounts payable system may reject or hold your invoice automatically.
- Government and public sector clients typically have strict procurement rules that require every payment to be tied to an authorised PO.
- Small businesses and individual clients rarely use purchase orders at all. They simply pay your invoice without needing a PO reference.
If your client sends you a PO, reference the PO number on your invoice. If they never mentioned a PO, do not worry about it — just invoice as normal.
What if a Client Asks for a PO but You Don't Have One?
Some clients — particularly those in larger organisations who are not familiar with how procurement works from the supplier side — may mistakenly ask you (the seller) to provide a purchase order. This creates confusion because you do not generate POs for your own work.
Here is how to handle this politely and professionally:
- Explain the situation clearly: "As the service provider, I issue invoices to request payment — purchase orders are typically created by your procurement or finance team to authorise the work on your side."
- Ask them to have their team issue you a PO number so you can reference it on your invoice: "If your finance team can send me a PO number, I will include it on my invoice for easy processing."
- If their organisation does not use formal POs, ask for written authorisation by email instead: "A quick email confirmation that you would like to proceed is sufficient — I will reference it on my invoice."
- This email authorisation effectively serves the same purpose as an informal PO and gives you something in writing if a dispute arises later.
Invoice vs Quote vs Purchase Order — All Three Explained
Three documents that are often confused, each with a completely different purpose:
- Quote (or estimate): Sent by the seller before work begins, showing the estimated or fixed price for a proposed job. Not a payment request — the client accepts or negotiates it before any commitment is made.
- Purchase Order: Sent by the buyer before work begins, authorising the purchase and committing to pay the agreed amount. The buyer's internal approval document — not a payment request.
- Invoice: Sent by the seller after work is completed (or at agreed milestones), requesting actual payment. This is the payment document. It starts the payment clock ticking.
In a typical corporate transaction, the sequence is: you send a quote → client approves and issues a purchase order → you do the work → you send an invoice referencing the PO → client pays. For most freelance work with smaller clients, the quote and PO steps are skipped entirely — you just send the invoice when the work is done.
Create Your Invoice — Reference the PO Number in the Notes Field
InvoFree makes it easy to add PO numbers, payment terms, and notes to every invoice PDF.