Ask for a receipt - it's to show that you have paid that amount. An invoice shows the amount that is requested - a bill.

An invoice is a request for payment. A receipt is the proof that you have paid. However I don't think that most shops would provide you with a receipt with your name and company on it. The receipt is typically produced by a cash register which automatically prints the amounts. An invoice would probably have those details. Invoice may involved data about your name and business which someone inputs into an accounting system.