visa gift card accepted for in-app purchase, then declined after I'd already received the credits

I recently purchased an iphone 7, my first real smartphone. I'm still learning the ins and outs of all the amazing things that I can now do, and it seems I've already messed up in a big way.

Last week, I made my first in-app purchases. I entered the information for a Visa gift card I'd had laying around forever and promptly received my goodies (very small game credits for a very small amount of money). Fast forward to today, and I tried to use the App Store to upgrade several free apps. I was redirected to a message that there was a problem with a previous purchase, and I needed to re-enter my payment info. So I re-entered the Visa gift card, only to receive the same message.

I did some searching, followed all the steps recommended when you can't select "none" as a payment type, and it was only really then that I realized I had received those small in-app purchases using a card that wasn't supposed to work in the first place. I have no other credit card, no debit card, and I never would have moved on those game credits if I had realized at the time that my gift card wasn't acceptable. Now I can't remove the card, I can't pay for the credits I've already used, and I can't update or download any apps. What's a girl to do?