If a borrower returns a car early, they'll still pay for the unused time remaining, 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 your trip summary 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 trip summary (the time you actually had the car) and the unused time charge, you'll see that you have only been charged for the amount of time you originally booked. You pay for the time you book upfront at the time you make the booking, so there's nothing more to pay if you return the car early.
If another borrower books the car during the unused time, then the owner hasn't lost any income and the borrower will not be charged for that time. The amount you pre-paid for this time will be refunded to your member account. It'll then be refunded to your card within a few days, or you can withdraw it back to your credit or debit card anytime. It can take a few days for your bank to process the refund and the money to appear back on your card.