They (National as well as all other rental car companies) bill you for the gas used if you return less than full (and did not buy the fuel option) at a rate of somewhere around 2x the average local price per gallon (close to $8 these days).

If you pay for the fuel option - basically you are paying for all the fuel in the car - the rate is slightly less than the average local price - they are counting on you returning the car with "some" fuel in the tank thereby making a profit on "selling" you the fuel at a slight discount.

There is no way for you to prepay for a few gallons (or some fraction - say 1/4 - of the tank).