I believe it depends on the detail of the booking. In at least one instance I tried to rent for less than 24 hrs and it wasn’t possible — there was a minimum.

After a certain # of hours, it is a daily rate, regardless. On the other hand, when I’ve picked up later than the expected time, the fees have always been by the clock so the return time plus 29 min was calculated from checkout time.

If she keeps it for less than 24 hrs and is charged extra, I would ask for a manager. Less us know what happens.