Still, you will be way overpaying the fuel if you let Hertz deal with it.

1) top off - you are paying a $6.99 fee
2) purchase in advance - sure you get the 15c discount, which is nothing in today's $4+/gal fuel price, but ask yourself, how often do you use up an entire full tank on a rental or return the car exactly as it is fully empty?

Hertz has plenty of opportunity to make money from this trap.