A person who returns the car with 3/4 of a tank will pay around $15. A person who filled it up would probably pay around 3.25 or around $9.75. The "penalty" for not taking the time to fill up the car is around $5.00
Under almost any scenario, short of running out of gas while waiting to return your car, the pre-paid gas plan will cost you more then the $5.00 extra you'll pay if you return the car at less then empty. Light driving you'll only use 1/4 tank. Otherwise just gas once.
I have no problem gassing up on the way to the airport but even if I did the pre-paid gas plan is still a bad solution.
Yes, Lewis, for you it is a bad solution, for others, it is a convienance that they are willing to pay for. Your numbers work well, but, consider one of those ginormous SUVs, a quarter of a tank of gas for that thing will cost a bit more than your 15.00 dollar figure, just sayin. I was floored when I saw the cost to fill one of them. Definitely makes me feel better about filling up my little 4 cyl. Honda Accord.

As I said before, I offer it and if they decline, I say Ok, tell them where to get reasonably priced gas, tell them what stations to avoid, ask if I can help them further and wish them a happy vacation and safe driving.