A receipt is the payment slip you received when purchasing the gift card, showing how it was paid for.
For most gift cards, including iTunes and Steam, a receipt is not required.
For Amazon gift cards, select the receipt type that matches what you have:
Cash or debit receipt: best rate
Credit receipt or no receipt: lower rate
Please note: Selecting the wrong receipt type will result in a rejected trade. See: What is the difference between cash, debit, and other Amazon receipts?