1. Purchase Order A Purchase Order (PO) is prepared by a customer and sent to us to order goods or services. The term "Purchase Order" has a special legal meaning under the Uniform Commercial Code (UCC) and implies a huge volume of terms and conditions that are set out in the UCC.

2. Packing Slip A PO is technically a legal offer to pay us a certain amount for the goods ordered. We can turn the PO into a contract by accepting the offer. The simplest way to accept an offer is to perform on it, in this case by shipping the goods. We prepare a Packing Slip as a record of the shipment.

3. Invoice An Invoice is a demand for payment. The invoice is sometimes legally necessary but also shows the customer the calculation of the total.

4. Receipt A Receipt is acknowledgement of payment. Receipts are usually only prepared when the payment is in cash; in other cases the customer’s canceled check serves as the receipt.