Is there a way to pre-pay (or be billed, upon returning a car) for just a few gallons of gas, rather than a whole tank?

I regularly rent cars in Charlotte, and I fill up on cheap gas in SC before crossing into NC and driving about 30 miles to the airport, which means that I have to buy about a gallon or so of gas right near the airport to refill the car. I'd like to avoid having to buy gas a second time in NC.

I thought about doing the upfront fuel option, but I see that it requires that you pay for a whole tank of gas.

The National website states that you'll have to pay for gas needed to fill up your car, if you don't return it with a full tank. That's fine, if I'd be billed for just about what I used up (a gallon or so)...but in practice, is the billing accurate?

Thanks.