Billed for refilling on a full tank

I hired a car from Avis for 3 days and am a bit shocked by the level of slyness that I have received from a so called reputable company

On my invoice that I received after I returned the vehicle I was charged for 4.37 litres of petrol. I most definitely filled the car right before I returned it. The petrol station was only about 100m from Port Elizabeth airport and would definitely not have resulted in me using 4 litres of petrol. Furthermore the car only has a tank capacity of 35 litres, thus I would have had to use approx an eighth of a tank, which was definitely not the case.

Furthermore I even asked the inspector about the petrol when I returned the car and he informed me that all was good and marked that it had been returned with a full tank. Finally I further asked the lady to confirm at the front desk that all was in order in terms of charges for fuel and damages etc and she informed me that all was in order.

I emailed straight away after I received the invoice stating that they have made a mistake and that they should not debit my account, but they went ahead and debited anyways. I have yet to receive any feedback from them and have tried several times to contact Avis but with no success