CHARGED FOR UNUSED FUEL - AVIS RENTAL THROUGH BOOKING.COM

I booked an AVIS Rental Car Through Booking.com for a fee - I return the vehicle and then get charged a further R1,115=33 by AVIS for extra items - including a FULL TANK of Fuel, when I only drove the vehicle for 197km's -- AVIS branch has said because I "paid for fuel upfront" there is no refund on the un-used fuel - Booking.com has Not responded to any of my queries, going back 2 weeks now - I am Not one bit impressed with either AVIS or BOOKING.com - ********* and unacceptable practice