*****ing my money

I returned the rented vehicle to Avis Loveday. The rental agreement was then printed and an amount of R54.99 was stated for fuel cost. Avis then proceeded to debit me R110. Which was double the amount. When I queried it the branch manager proceeded to tell me that the amount of R54.99 was an estimation by the system. How is this my problem that the system underestimates the amount. No where on the agreement does it state that the amount quoted is an estimation. This is ****************. Avis is *****ing my money. I am so disappointed.