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Purchase order vs invoice vs packing slip
Three documents, three different jobs. Most purchasing problems that look confusing are really just two of these three not agreeing with each other.
Each of these documents is created by a different party, at a different point in the order, and answers a different question. Confusing them is where a lot of avoidable back-and-forth with suppliers comes from.
Purchase order (PO)
- Created by
- Created by you, the buyer
- When
- Before anything ships
What did we agree to buy, at what price, and by when?
The PO is the request and the commitment. It's what you compare everything else against later. Once a supplier accepts it, both sides are working from those terms unless something changes.
Packing slip
- Created by
- Created by the supplier
- When
- Included in or attached to the shipment
What's actually in this specific box or delivery?
A packing slip lists what was physically shipped, which isn't always the full order. Partial shipments, substitutions, and backorders all show up here first, often before anyone tells you about them directly.
Invoice
- Created by
- Created by the supplier
- When
- After shipping, sometimes before, sometimes after delivery
What do we owe, and by when?
The invoice is the bill. It should match the PO's pricing and the packing slip's quantities, but it's a separate document from both, generated by the supplier's own billing process.
The three-way match
Accounts payable teams call this the three-way match: compare the PO (what you ordered), the packing slip or receipt (what actually arrived), and the invoice (what you're being billed) before approving payment. When all three agree, payment is straightforward. When they don't, that gap is exactly where a follow-up email is warranted, before the invoice gets paid for something that wasn't fully delivered.
The most common mismatches: quantity shipped doesn't match quantity ordered, invoiced price doesn't match the PO's agreed price, or the invoice arrives for an order that hasn't been received at all yet.
Why this matters for follow-up specifically
A late delivery and a pricing discrepancy need different emails. If a packing slip shows a partial shipment against a full PO, the follow-up should ask about the remainder, not repeat the whole order. If an invoice doesn't match the PO price, that's a billing question, not a delivery one. Knowing which document disagrees with which tells you what to actually ask for.
Keeping the PO side of this current
The PO is the reference point every match starts from, which only works if its status and promised date stay accurate. See the status guide for how to keep that side maintained, or explore how SupplierPing does it automatically.