The store is sometimes bugged when your balance ia bit higher than the script price. When buying a gift voucher makes sure to always get the exact balance to prevent his.
What you can do to solve this is buy a gift card of a few cents yourself so you balance matches what you need
https://osbot.org/forum/store/gift-cards/
Use "Print" and enter the amount you like to get rid of you can claim it again in the end