I just went to try to buy a new character slot, and when I went to the checkout screen, I got an "Error 004" message, saying I should try again in a few minutes. Its been this way for the last 30 minutes (as far as I can tell, from when I first tried to buy).
The pop-up for choosing between Visa/Mastercard seems to be busted, and I'm guessing that's related to the issue. A few friends also tried the checkout but got the same error.