Money deducted for no reason, after I filed up car

I rented a car through Avis to travel in Namibia from the 22 Dec until the 26th. As I've received Namibian dollars for Christmas I decided to use it full up the rental before returning it as I can't use Namibian dollars back home in SA. I returned a practically full car (I fulled up in Windhoek, which is a 45km drive from the airport.) And the car was also cleaned before I returned it. I was told by the inspector all is fine, and I can just leave the petrol slip in the car which I did.<br> <br> Days later R485.61 was deducted from my credit card by Avis. I never received an invoice with a cost breakdown or anything. I spoke to Vivian Sibanyoni in customer care, and he said 276.80N$ was deducted for fuel. How can a fulled up Polo Vivo use that much petrol over 45km? And how do they get to a total of 485.61? Customer serves said the amount was for outstanding fuel, and I had to show the petrol slip as proof. I find this unacceptable as I think they either mixed my bill up with another car, or someone is trying to cheat me since I never even received an invoice.