If you return a car early, you will still be charged for the unused time remaining in your booking.
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 your invoice as an 'unused time charge', but it's not an extra charge or fee for returning the car early. If 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 full amount of your original booking time.
If another member books the car during your unused time, then the owner has not lost any income and so you will not be charged for that time.