Differences

Four documents that get mixed up often

Each has one moment in the deal, and sending it outside that moment confuses the other side.

The quote

A proposal with a validity period. It binds nobody, which is why it carries the date it expires.

The purchase order

Approval issued by the buyer. It turns the quote into a commitment and is referred to by its number in the rest of the paperwork.

The invoice

A demand for payment. It carries the amount and the due date, and it goes to accounts.

The receipt

Proof that payment happened. It follows the invoice rather than preceding it, and that is the order most often reversed.