After peeking at this, I think it needs to be specced out more. A few details seem like potential rabbitholes, and would be good to work out ahead of time.

• The iDEAL form shouldn't show sibling rtbt_* submethods, this begs the question of issuer as a sub-submethod, and gives us a good reason to generalize PaymentMethod and use it in an opaque manner.
• Show iDEAL logo, or any method's logo when it is the only available method. Probably, layout is often determined per method.
• ...