I would have to assume you only linked the PlayOnline Account to a Square Enix Account previously.
That is not the same as the transfer, which they are now finally pushing mandatory for everyone.

I can not see the system failing so badly that one even COULD do it twice.
Everything is possible, of course, and I do understand your frustration very well!

For what it is worth, I have used the Crysta-nonsense method since the transferral first became available and have not had any problems with payments, or any other account management stuff.

I know there are some less fortunate users, though. =/


Just some thoughts!
Better luck for the future~