There's no toll between I-59 and I-45 on the Sam Houston Tollway. Depending on where you are, 59 is often faster. Since it seems you want to save $, do not fill up at the Shell-For-Suckers on JFK at Greens. U-turn at JFK and Greens to the Exxon at the same intersection (I think IAH planted bushes in the median to hide it). Shell consistently gouges an extra 50-60 cents/gallon since they are on the rental center-bound side of JFK. From the Exxon, it's less than a quarter mile south on JFK to an easy U-turn to head back north to rental car return, or exit onto Greens and turn left onto JFK.