When an order is completed, a receipt is sent to the email address on your account. If you aren't receiving receipts, check the spam or junk folder of your email address associated with your account.
To review past receipts, you can also do the following:
1. Tap the profile icon at the bottom of the main app screen
2. Select "ORDERS" from the menu
3. Scroll down to review past orders
4. Tap "VIEW RECEIPT"