Yeah, they always say that but I've never had them check a receipt when I turn in the car. They are usually way too busy checking in 10 other cars that arrive at the same time. They'll check to see if the gas tank is full but I've not had them ask me to produce a receipt or even ask me where I filled up.
But, there are a few gas stations off Semoran Blvd right before you exit for the airport. The first two across from each other (one is Suncoast Energy, the other one I can't remember its name), skip those. They are always double the cost. Not kidding. Pulled into Suncoast and they were charging over $5 a gallon. Pulled right back out without pumping anything. Drive on down the road a half mile or so, maybe a mile and there's a 7-11 and a couple of others charging the normal, going rate for gas.
There are Hess stations all over that area too.