Poor customer service, and poor correspondence

I had rented a vehicle on the 14th of August 2015 at approximately 9:30am at the AVIS branch at King Shaka International Airport. The agreed return time of the vehicle was 9:00am the following day (15th August 2015). As per agreement, I returned the keys to the car at 9:10am at the AVIS branch at Oribi Airport, with no defects and a full tank of petrol. I was informed by the assistant that I only needed to drop the keys off. I then made a follow up call to confirm the return of the deposit, but was informed by a Call centre Agent that I was being billed for an additional day, as they had completed their inspection at 10:31am. I have no control after the time that I returned the keys, nor do I have any control over when AVIS does their vehicle inspection. I was informed that AVIS will not credit me for the \additional day"charged. I am disappointed with the service of what I thought was a reputable institution"