So you can’t charge RL money for your application but you can however charge isk but you cannot automatically take payments from accounts, quite rightly so.
Here comes my idea, wouldn’t it be good if there was a way to setup “direct debits” between users that the ESI can trigger. Recurring automated monthly payments. These could also be used by renters and anyone else that sells things based on subscription via in game settings but the main motive behind it is to allow developers to charge isk subscriptions in a safe manner for the dev and the user.
Using the already available UI end points the dev can prompt the user to create a “direct debit” giving the amount and the frequency of the payments as well as outlining where the ISK will be paid. The user can also find and cancel these on another screen however the key being the ESI then gets an endpoint to trigger this getting a status back. This would allow the game to only allow a success when it should do an reject the request all the other times. This would allow developers to request the money and update subscript status automatically and if the transfer fails the service can be cancelled, much like in game mechanics.