Edit an open-source code

Hello, I would like to get a tool (kind of). There's this wallet [login to view URL] and it's open source. In that wallet there is an option to buy amazon gift-cards directly through that app. Thing that I don't like about it, is that the payment for the card is done from the funds that is in that wallet, it doesn't let me pay from any other wallet (say Bitcoin core). But I think it's possible to make it so, because for every card purchase there is a new invoice (that can be viewed on web) created it's just that it's not shown in the app (only after it's paid). So what I ask you to do is to create a tool (or even better to edit the existing code) so that after I enter the amount of how much I want to purchase, it would show the link of an invoice instead of instantly paying for it regardless of how much money there is in the app. So for example if there is 0 in my wallet I could still create an invoice of 10$ and after the invoice is paid the code of the card would appear in the wallet.