Read the contract, check all the options, beware what you sign

I obtained a car from Avis March 2019, opting for all the waivers. The attendant asked about the fuel, should they refuel on return of the car, or will I do it. I asked them to refuel the car, add the used fuel to my account. This is the way I have done on numerous occasions, the company adding the used fuel to my account. What I didn’t notice was an entry called “ fuel up front option”, where they add a full fuel tank to your account, whether you use it or not. I The fuel gauge stood at three quarters when returning the car, but I was charged for a full tank, not for fuel used. The rationale behind this escapes my reasoning, except that every time the car is returned with fuel in the tank, Avis scores, the same fuel is sold over and over to customers. Talk about easy money!!