Paypal please! (or Riot :P)

[DSX]

If there is an error while trying to pay then you should wait for at least 24 hours
because if you try again right away and the same error occur then you get locked out for a certain amount of time (I think 24 hours initially)

If you try again while you are locked out then that ban period get doubled
If you try again then the ban period get doubled again

so you either have to wait for the ban time to expire (they don't tell you how long is the ban in the site) then try again to pay or you contact them and if you convince a helpful agent of your case they will remove the ban.

That is how the system works (at least how I think it does).

I also support the idea of selling crysta cards.