Can't Buy Supporter Pack via Upgrade/Credit Card Error?

Ahoy! I'm trying to buy one of the supporter packs by upgrading from previous point purchases, but I keep getting an error (Code 500) when I try to enter my credit card info to make the actual purchase. Anyone know what this code means and/or what's causing it? Unless GGG has a huge fee there should be enough to make the purchase on the card. It's a prepaid card if that affects anything. Thanks!

Never mind. Found an old post that suggested buying the $5 pack, saving the credit card info, and upgrading using the saved info. That worked.