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 op

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.)