I've been having the same problem with domestic Brazilian bookings (on the US website) for the past few days trying with multiple cards. Also, one's desired and selected seat disappears for several hours as part of the process, so you can't try various cards in succession if you want a specific seat.

Maybe I'll throw in the towel and try the PayPal option the next time and leave the few hundred miles for the credit card spend on the table. (Update: I actually just tried PayPal too; that didn't work either.)

Regardless, it's both weird and annoying that there's not on the ball with this issue. Oh well.