Good points. About validation step, how about a notification that counts down and auto accepts unless you cancel or something? Never thought about it before but having a guy in a random skeezy mini mart say "3.75" then I just tap and it is done without me every seeing if he ripped me off seems sketchy. And that is more sketchy than most CC machines because usually it actually does ask you to confirm the amount which google pay does not.
Exactly. Buying with a card is like handing over your wallet for the cashier to fish out the correct amount; a new system using a phone should be taken as an opportunity to put you back in control. The only friction would be one tap to confirm. The tap could double as your action of choosing which card to pay with.
(Confirming on the vendor's reader still requires you to trust their machine, not yours.)
At least with my configuration (because I'm really not sure which parts of this are bank-driven, which are OS-driven, which are app-driven, et cetera - it's a bit of a mess) I get a notification telling me how much I paid and to where, which can be dismissed at some later point.
