Yeah, that's kind of a large oversight. There are ways to work it (pay with the retainers with a different payment type, cancel the retainers for a day when your subscription recurs then add more crysta and reactivate), but they shouldn't be necessary.

Silly of them to make it hard to take your money. I doubt you'll get a decent answer this early on, but you might want to contact support and see what they say about it. At least it's another avenue for making SE aware of the problem.