Obviously the fair cost should be amp hours needed to charge the battery times cost of electricity where the vehicle is charged.
> This customer actually returned the car with a 96% charge, the same as they’d picked it up at.
The price should be simply accounted for in the rental price.
In the EV case it takes a significant amount of time to charge the vehicle (nobody wants to do that before taking a flight) and it is more expensive for the user in a public charging station than it would for a rental company that invested for its own chargers within their parking spots. Also the time used to charge the EV can be mutualized with the time it is eventually checked and cleaned. Very rarely is a car returned and rented within minutes.
I guess in an ideal setup you would get a car with at least 60% of charge and when you return it the rental company expect the same. This leave you time to plan your trips and charge so you don't have to look for a crappy place to do a last charge with nothing else to do just before returning it.