An Update:
I managed to get through to someone and they cancelled my appointment, but the confirmation email says that my 'refund amount' is $63? I was under the impression it would be free, because it was a payment processing issue, but it seems they're treating it as a voluntary cancellation on my part instead.
Does anyone have any tips on what I should do from here? I've heard it can be difficult wrangling with GMAC over money stuff.
EDIT: To clarify, I'm worried GMAC will make me pay the remaining amount. As the original payment never went through, no money has been exchanged yet at all.