Wrongfully Charged - Avis

We had a terrible experience with Avis! My boyfriend hired a Renault Kwid from Jhb Airport and we took it on our 5 day trip to Kruger. We hired the car with the Windscreen Tyre Damage Waiver activated in case anything had to happen. When we got to Jhb (before we even saw the car) the lady at the front desk told us that the car had a chip in the windscreen which they already new about. My boyfriend signed the contract which showed there was a chip in the windscreen, we asked for a copy of the contract to be sent to us (which it never did) and the lady who did the inspection before we took the car took photo's of the chip in the windscreeen. Then on our return to Avis, we told them that the chip was already there and they took photos again of the chip on the windscreen and were told it was all sorted, only for us to see that we got billed for the repair of the windscreen! We are sure that they saw that we took the Windscreen Tyre Damage Waiver and they deliberately gave us a damaged windscreen so that they can charge us and get it repaired. It occurred on the 20th November and it is now the 12th December and we have had no clarification or help from them, it was also charged off our credit card. I will not use them again and will tell our family and friends of our experience!