Have you tried turning it off and on again?
Actually, clearing your cache might help the problem, also, I have a hunch that Xsolla will only use your credit if you are buying more than what that credit is worth, so maybe try and buy $25 and then it'll show up?
Only guessing, but I'm basing it on my own transactions, two of which had store credit on them, and both of which subtracted the store credit automatically leaving me with my actual payment.