I'd be quite interested in the billing part.

Such a charger needs to be presented some kind of authorization, but the amount of energy transferred (and thus the amount charged) is only determined later.

Is the logic for that server-side, or is it triggered from within the charger? If the latter, could you avoid being charged by power-cycling the computer inside the charger before finishing the charge?

Probably the safer option would be for the charger to send a message like "for authorization XYZ, I've, until now, supplied 5.003kWh" every 5 seconds, then a server-side stream processor can detect the end of the charge and initiate billing.