i've got this same problem before,
it goes like this,
i'm using the ambank nextG perpaid credit card,
lets say i've got credit enough for 1 month and i subscribe for 1 month,
next month they will still bill it into your credit card even though there isn't credit inside, and the amount you owe MS will be deducted immediatly the next time you top-up,
they deducted 3 months of fee from my card when my console got 3rod...damn..