Get Charged extra for taking back car early

Rented a car from them on the 26 Feb 15, we had paid all the necessary charges and had confirmed so over the phone. A couple of days later we see they had charged us an extra R650.55 on top of the agreed fee. Called to ask why and was told that the extra charge is for returning the car earlier than what was agreed. So wait....what?? You get charged extra for getting their car back to them a day early......although you had paid for two days, you get charged extra for returning the car in one day..... SURE because that makes a whole lot of sense. This always happens when companies get to keep your card details & just charge you for anything they feel like. They usually just do this as they see fit and don't ever notify you in any way.