The prepaid option is paying upfront for a FULL TANK of gasoline regardless of how much is in the tank on return. The second option is to fill the tank yourself before you return the vehicle. In Hawaii, if you buy the gas package, but fill the tank just before you return the vehicle, and can show the receipt (it must be from a gas station within 10 miles of the Car Rental lot, and within a few minutes of the car return time) you won’t be charged for the gas package.