Users charged even though they have enough Credit

[ItsNeil]

Hello,

 

I recently ran a giveaway for store credit on my host, and when the winners (with enough credit) went to check out, they had to add a payment method, when they tried to link their accounts, they were charged. Luckly, this person didn't have enough funds in their account, so no money was taken, but still, it's an issue. How can I fix this?