Avis card rental - beware of extra charges and fuel charges

We rented a vehicle from Avis at Cape Town International airport and chose the basic agreement with no extras. Rental from 12 to 14 October. We returned the vehicle on 14 October and filled up the car at the garage at Airport and informed the AVIS staff. On 14 October we received and agreement with additional charges of R 261.75 and queried it. On 16 October more money was deducted - now for extra fuel cost and we gave them proof of refuel and km travelled was a 100% match. Requested proof that they filled up the car R 346.86 and also additional charges. Total extras of R 601.61 and no explanation. 15 emails later, without proof of extras and fuel, what are they hiding? Seems like they fill up their own cars?