I am offering gift cards on the website. The only option

Hello, I am offering gift cards on the website. The only option for the card is that they can choose an amount. I have been getting a lot of re-orders and most of the time it is not necessary to send them a new card when I can reload their current card. This also makes it so they can use the card sooner and not have to wait for a new one. I want to add a field to the product for their existing card number. How can I do that in marketpress?