For whatever reason if you drive more than so many miles (differs per car) then they charge you a flat rate for a full tank. They screw that up sometime, though. I returned a car to HNL and they charged me $84 even though I had the gas option. I went straight into the office a disputed the amount and they cut $50+ dollars off the amount. I ended up paying less than I would have if I had to fill the tank up (I returned it near bone dry) myself.