Purchases made in the Apple App Store are recorded in Apple's billing systems. If you need to find an invoice/reciept, start by selecting this link to go to Apple Support. Once on the Apple Support screen, select View your purchase history (see below).
You'll come to a page that lists the roster of your purchases on the lower half of the page (blurred for privacy).
Scroll through the listing until you identify your purchase. Then select the order number (circled in red below).
Finally select View Receipt. You'll be able to review or screenshot for your records.