No they don't, because I returned it before a shortage occured. They just lost the profit from rent.
That's strange, the goal post was right here just a minute ago...
Whatever time period you had taken the rental car for was a time period that they could not have rented it to someone else. You can't know until after the fact if there would have been a shortage in that time frame. There's also the extra wear you have put on the car.
If you took a rental car for 5 minutes and returned it I doubt anyone would charge you with anything.