Invoice vs Purchase Order: What's the Difference?

If you're navigating the world of business transactions, you've likely encountered both invoices and purchase orders. While they're both essential business documents, they serve completely different purposes and come from opposite sides of the same transaction.

Here's the quick answer: A purchase order (PO) is created by the buyer before a transaction to request and authorize the purchase of goods or services. An invoice is created by the seller after delivery to request payment for those goods or services. The buyer initiates with a PO; the seller responds with an invoice.

Understanding this distinction is critical for maintaining accurate financial records, ensuring smooth payment processing, and avoiding confusion in business relationships. Whether you're a small business owner issuing your first invoice or an accounts payable clerk trying to match documents, knowing when to use each document—and how they work together—will streamline your operations and prevent costly mistakes.

This guide breaks down everything you need to know about purchase orders versus invoices: what each document is, who creates them, when they're used, and how they work together in the procurement-to-payment cycle. By the end, you'll have a clear understanding of both documents and when your business needs each one.

What is a Purchase Order?

A purchase order is a commercial document created and issued by a buyer to a seller. It represents a formal request to purchase specific goods or services at agreed-upon prices and terms. Once the seller accepts the PO, it becomes a legally binding contract between both parties.

Think of a purchase order as the buyer saying, "I want to buy these items from you, here's what I need, and here's what I'm willing to pay." It's the buyer's way of formally authorizing a purchase and creating a paper trail for their procurement process.

Key Characteristics of Purchase Orders

Buyer-initiated: The purchase order originates from the customer or client—the party who wants to acquire goods or services. The buyer's purchasing department typically generates POs as part of their procurement workflow.

Created before delivery: POs are issued before any goods are shipped or services are rendered. They're part of the planning and authorization phase of a transaction, not the payment phase.

Authorization document: A purchase order serves as internal authorization for spending. In many organizations, purchases above a certain threshold require a PO to ensure budget approval and spending oversight.

Reference number: Every purchase order includes a unique PO number that both parties use to track the transaction throughout its lifecycle—from order placement through delivery and eventual payment.

Essential Elements of a Purchase Order

A complete purchase order typically includes:

• PO number - Unique identifier for tracking
• Buyer information - Company name, address, contact details
• Seller information - Vendor name, address
• PO date - When the order was issued
• Item details - Description, quantity, unit price
• Total amount - Expected cost of the order
• Delivery date - Expected or required delivery timeframe
• Shipping address - Where goods should be delivered
• Payment terms - When payment will be made (e.g., Net 30)
• Special instructions - Any specific requirements or conditions

For sellers, receiving a purchase order is valuable because it represents a commitment to purchase. You can confidently fulfill the order knowing you have documentation of what was requested and the agreed-upon terms. It also provides the critical PO number you'll reference on your invoice to help the buyer match your bill to their original order.

What is an Invoice?

An invoice is a commercial document created and issued by a seller to a buyer. It itemizes the goods or services that have been delivered and requests payment according to agreed-upon terms. An invoice is essentially a bill—the seller's formal request for money owed.

If a purchase order is the buyer saying "I want to buy this," an invoice is the seller saying "I delivered what you ordered, now please pay me." It's the seller's accounting record of the transaction and the buyer's notification that payment is due.

Key Characteristics of Invoices

Seller-initiated: The invoice originates from the vendor, service provider, or seller—the party who provided goods or services and is now requesting compensation.

Created after delivery: Invoices are issued after goods have been shipped or services have been completed. You can't invoice for something you haven't yet provided. The timing depends on your payment terms—some businesses invoice immediately upon delivery, others at regular intervals (monthly, for example).

Payment request: An invoice's primary purpose is to request payment. It tells the buyer exactly how much they owe, what they're paying for, and when payment is due.

References the PO: When a purchase order was issued, the invoice should always include the PO number. This allows the buyer's accounts payable team to match the invoice against the original purchase order to verify that what's being billed matches what was ordered.

Essential Elements of an Invoice

A professional invoice typically includes:

• Invoice number - Unique identifier for the seller's records
• Invoice date - When the invoice was issued
• Seller information - Your business name, address, contact details
• Buyer information - Customer name, billing address
• PO number - Reference to the buyer's purchase order (if applicable)
• Item details - Description, quantity, unit price, line totals
• Subtotal - Total before taxes
• Tax amount - Applicable sales tax or VAT
• Total amount due - Final amount the buyer owes
• Payment terms - Due date or terms (e.g., Net 30, Due upon receipt)
• Payment methods - How the buyer can pay (bank transfer, check, card, etc.)

The invoice serves multiple purposes: it's your official request for payment, a record for your accounting books, documentation for tax purposes, and a legal document establishing the debt. For buyers, it's what triggers payment processing and serves as proof of the transaction.

Key Differences Between Purchase Orders and Invoices

While purchase orders and invoices are both critical business documents, they differ in fundamental ways. Understanding these differences helps you know when to use each document and how to properly process them.

Who Creates the Document

Purchase Order: Created by the buyer (customer/client). The purchasing department or authorized buyer issues the PO to vendors they want to purchase from.

Invoice: Created by the seller (vendor/service provider). Your accounting or billing department issues invoices to clients who owe payment for delivered goods or completed services.

When the Document is Created

Purchase Order: Created before the transaction occurs—before goods are shipped or services are performed. It's part of the planning and authorization phase.

Invoice: Created after the transaction is completed—after goods have been delivered or services have been rendered. It's part of the payment phase.

Purpose of the Document

Purchase Order: Authorizes the purchase and creates a binding agreement. It says "I want to buy this from you at this price under these terms."

Invoice: Requests payment for goods or services already provided. It says "I delivered what you ordered, please pay me this amount by this date."

Legal Status

Purchase Order: Becomes a legally binding contract once the seller accepts it. Both parties are obligated to fulfill the terms—the seller must deliver, the buyer must pay.

Invoice: Serves as a request for payment and legal proof of the transaction and debt. It's evidence of what was sold and what's owed.

Information Flow

Purchase Order: Flows from buyer to seller. The buyer sends the PO to the vendor to initiate the transaction.

Invoice: Flows from seller to buyer. The vendor sends the invoice to the customer to request payment.

Document Timeline in a Transaction

Here's how POs and invoices fit into a typical procurement cycle:

1. Buyer creates purchase order - Specifies what they want to buy
2. Seller receives and accepts PO - Agreement is formed
3. Seller delivers goods/services - Order is fulfilled
4. Buyer receives goods/services - Delivery is confirmed
5. Seller creates invoice - References the PO number
6. Buyer receives invoice - Matches it to PO and receipt
7. Buyer approves and processes payment - Transaction complete

Quick Comparison Table

The key takeaway: These documents work together as a checks-and-balances system. The purchase order sets expectations upfront; the invoice confirms what was actually delivered and triggers payment. Together, they create a clear audit trail and prevent disputes.

How Purchase Orders and Invoices Work Together

Purchase orders and invoices aren't competing documents—they're complementary pieces of a well-functioning procurement and payment system. When used together properly, they create transparency, prevent errors, and streamline payment processing.

The PO-to-Invoice Matching Process

When a buyer receives an invoice that references a purchase order number, their accounts payable team performs a matching process before approving payment. They compare the invoice to the original PO to verify:

• Quantities match - Is the seller billing for what was actually ordered?
• Prices match - Do the unit prices on the invoice match what was agreed in the PO?
• Terms match - Are payment terms consistent between documents?
• Calculations are correct - Do line items add up to the correct total?

If everything matches, payment is approved. If discrepancies exist—perhaps you billed for 100 units when the PO specified 75—payment may be held up while the issue is resolved.

Three-Way Matching

Many organizations use an even more robust control called three-way matching, which compares three documents before releasing payment:

1. Purchase Order - What the buyer ordered
2. Receiving Report - What the buyer actually received
3. Invoice - What the seller is billing

This prevents situations where a vendor bills for items that were ordered but never delivered. If all three documents align, payment proceeds. This process protects buyers from overpayment and fraud while giving sellers confidence that properly documented deliveries will be paid promptly.

Why PO Numbers Matter on Invoices

When your client provides a purchase order number, always include it prominently on your invoice—typically near the top, alongside your invoice number and date. This simple step:

• Speeds up payment - Accounts payable can immediately match your invoice to the PO
• Prevents payment holds - Many organizations automatically reject invoices without PO numbers
• Reduces disputes - Clear documentation prevents confusion about what was ordered versus billed
• Improves professionalism - Shows you understand proper business processes

Common Workflow Example

Here's how POs and invoices work together in practice:

A marketing agency needs design work. They issue PO #12345 to a freelance designer for 10 custom graphics at $100 each, total $1,000, Net 30 terms. The designer accepts the PO and completes the work. After delivering all 10 graphics, the designer creates Invoice #INV-2025-001 billing $1,000, with "PO #12345" clearly noted at the top.

The agency's accounts payable clerk receives the invoice, looks up PO #12345, confirms the quantities and prices match, verifies the graphics were received, and approves payment. Because everything was documented and matched perfectly, payment is processed within the Net 30 terms. No delays, no disputes, no back-and-forth.

When is Each Document Required?

Not every business transaction involves both purchase orders and invoices. Understanding when each is required—or optional—helps you adapt to different client requirements.

Purchase Orders

Typically required by:

• Large corporations with formal procurement departments
• Government agencies and public sector organizations
• Healthcare and educational institutions
• Companies with strict budget controls
• Organizations subject to financial audits

Often optional for:

• Small businesses without procurement departments
• One-person operations and sole proprietors
• Freelancers and independent contractors
• Low-value or routine purchases
• Subscription services and recurring transactions

Many small businesses operate with invoices alone, especially for service-based work. They'll contact a vendor, agree on scope and price via email or contract, receive the service, and then receive an invoice. No formal PO is issued. This streamlined approach works well when transactions are straightforward and both parties trust each other.

Invoices

Invoices are required in virtually all business transactions where payment is expected after delivery. Whether or not a PO exists, you should always issue an invoice when you:

• Complete work for a client
• Deliver goods to a customer
• Provide services under a contract
• Need formal documentation for tax purposes
• Want legal proof of the transaction

Even if your client is a small business that doesn't issue purchase orders, you should still send a professional invoice. It protects both parties, ensures you get paid, and provides the documentation you need for accounting and tax reporting.

Best Practices

• Ask upfront: When starting a new client relationship, ask if they require a purchase order before you begin work
• Wait for the PO: If a client says they'll issue a PO, don't start work until you receive it—it's your protection that they've authorized the purchase
• Always invoice: Even without a PO, always issue a professional invoice to document the transaction
• Reference the PO: If a PO was issued, always include the PO number on your invoice
• Keep records: Retain copies of both POs (received) and invoices (sent) for tax and legal purposes

Conclusion: Two Sides of the Same Transaction

Purchase orders and invoices are mirror images of each other—one represents the buyer's intent to purchase, the other represents the seller's request for payment. While they serve different purposes and are created by different parties, they work together to create a transparent, documented transaction process that protects both sides.

As a seller or service provider, your primary focus is on creating accurate, professional invoices that reference any purchase orders your clients provide. This simple practice speeds up payment, reduces disputes, and demonstrates your understanding of proper business processes.

Whether you're dealing with large corporations that require formal POs or small businesses that operate with invoices alone, understanding these documents ensures you can adapt to any client's requirements and maintain smooth, professional business relationships.

Frequently Asked Questions