If a Borrower returns a car early, they will still be charged for the unused time remaining in the booking, up to a maximum of 12 hours.
This is to compensate the car's owner for the fact that your booking may have prevented others from booking the car. This will appear on the trip invoice as an 'unused time charge', but it's not an extra charge or fee for returning the car early.
If, as the Borrower, you add up the time charges on your invoice (the time you actually had the car) and the unused time charge, you'll see that you have just been charged for the amount time you originally booked.
If another member books the car during the unused time, then the owner has not lost any income and so the Borrower will not be charged for that time.