This is something you add support for as an optional convenience feature because you can do it entirely in software, not something you require people to use. You still need a card reader on the charger.

This is the fundamental problem with all of this stuff -- nobody wants your app. If you can't make it better than sticking a card in the machine, go away. If you can, prove it, by making it an alternative to sticking a card in the machine instead of the only option.