Failure to respond

On 2 occasions, in November and December I rented a car at Cape Town airport, both times paying in advance. On returning the cars I checked that everything was ok and was assured it was. I then received a charge of over GBP40 on my credit card. On emailing Avis I was Informed this was for windscreen waiver, which at the time I stated I did not want, and because I returned the car 2 hours late. On both occasions my return flight was brought forward, so the cars were returned 2 hours early! I was informed, by emai from Theodora Msibi on 21st December that Avis would reply before the end of the day. Today is 27th January, and despite my emailing several times since 21 December, and asking for a manager, I have had no response. I have had a bill from Avis saying the charges are for windscreen waiver and refueling. On both occasions I refueled prior to returning the cars. I have also received another charge of over GBP30 on my credit card. I would like the matter resolved.