Ripping customers off after renting cars

I rented a car from Avis on the 21st of August 2021 for 1 day, from O R Tambo. Knowing there are normally extra changes when returning the car, I ensured that I filled up the car at the Airport petrol station. When returning the car on the 22 of August, the person that had accepted the car back from me asked if I had filled up the car - of which I responded yes. Now, the 24th of August I have received my invoice and have been charged a full tank of fuel R823.50. What absurdity is this? How can a company of this magnitude rip customers off like this? How does Avis ever expect me to return as a loyal customer?