Don't you need to run a coin selection algo on the card. Is this possible with current hardware?
I think tap to pay from your phone is probably more achievable.
Discussion
Coin/ecash control would be dope
That'd be on the software side (app) imo
If you want to pay with a card and not an app you need to load some ecash onto the card. We can do this today.
Ecash is a bearer instrument so if you don't want to spend your entire balance you need a way for the card to only unload some of its ecash but not the rest.
You don't give the cashier the entire contents of your wallet when you go to the register. Same thing.
I think you need a processor on the card capable of running an algorithm. I don't know whether this is possible today.