Not sure if someone has mentioned this. I have seen where others have used the ultimate pay through other web sites, but how i did it was to go in like your adding Crysta select the ultimate pay option. There you will have three tabs. One for credit/debit, one for PayPal, and one for other. I selected the CC/debit there and it took my card to add Crysta. Then i applied the Crysta for my service account and it worked fine. All on the Square-Enix site to add Crysta then the Mog station to activate the account. This was also done right after i had tried to use my credit card on the Mog station to activate my service account. It is definitely something on their end, but i was able to do it easy enough without having to go to game stop or whoever for the ultimate pay card.
Just my 2 cents take it or leave it. But either way it is still something on your end Square, Should really get it figured out.