I don't see the obvious problems with the car paying. It can do exactly what google and apple pay does. Issue a one use token for payment to the charging system which is ultimately redeemed with a credit card purchase/money transfer. The only needed system is for either car manufacturers to setup their payment systems, or for them to rely on some 3rd party to provide the same system. There's already an android auto and apple auto so I'd imagine google or apple would both leap at the opportunity to handle payment for them.
The only way with obvious problems, to me, is a system that requires you to pull out a credit card. That is wide open for card skimmers.