Padding their profit at the cost of customers

We hired a car (ford fiesta) from Avis Port Elizabeth airport between 25 August and 1 September. We experienced no problems during the rental and returned the car at just over a 1/4 full of petrol on the 1 September. When the final invoice came, petrol for 45 litres were included. This is the actual petrol tank capacity of the car and therefore CANNOT be the true number of litres and therefore cost of petrol for the car. As I have had the same problem in 2016 after a rental with Avis, I queried this. In 2016, the invoice was corrected with the ( I hope) correct number of litres. This time after queries on 4 September and follow up on 19 September, I have had no feedback! 1) Do Avis regularly overcharge their clients on petrol? 2) Why is customer service so slow that in a month they have been unable to come back to me? 3) I have paid the full invoice - now give me back the money you took from me based on a false invoice!

Yolandé