AVIS RENT A CAR PETROL CHARGE

This is not the first unresolved issue I have had with Avis rent a car. We hired a Volkswagen Kombi on the 24 February 2020 from Skukuza Airport for international guests. The collection time was 8h30 but the gentleman only opened the office at 8h45 so we were running late for collecting our guests from the hotel. He quickly ran through the documents on the iPad and asked if I wanted to pay for a tank of petrol upfront. I asked if there was any difference and at no time did he explain to me that there would be no refund for any petrol not used. So I agreed, assuming that when I returned the car the tank would be filled and I would be credited the difference. I would NEVER had agreed to pay upfront if I knew that I would pay for unused petrol. The tank of petrol cost me R 1330.40 and we only traveled 324 km. This is tantamount to theft as any unused petrol is additional undeserved income for AVIS. I tried to resolve this with Paul from AVIS prior to lockdown but he never got back to me. I have since called AVIS but there is no answer. I have also e-mailed them on two occasions but I have not even received acknowledgement of my mail.