How not to treat Clients

I am still waiting on Avis to give me feedback as to why they charged me for petrol in a car I returned to them on the 11 December 2014. I filled it up before returning it. I was then charged for a FULL tank of petrol.<br> After numerous emails and telephone calls, they refunded me part of the refuelling charge. After further arguing, they refunded the balance, with this 'disclaimer' '....giving the benefit of the doubt'.<br> I told them that I would not accept that they were doing me a favour as I have the petrol receipt. They promised to get back to me on the 13 January 2015. I have made 2 calls since, still no resolve.<br> This in my opinion is an ongoing practice of Avis. If you query it, they will do something after a fight. I STILL WANT ANSWERS.<br> When you return a car to Avis, make sure that you check the petrol level in the tank. Make sure that you know how many litres that tank holds.