Maybe petition the Hertz and Thrifty locations to erect signs warning of the gas station across the street? They could even use it as part of their sales pitch for prepaid fuel ("Now, the gas station across the street charges $4.XX per gallon, but we'll sell it to you for only $2.XX")

Interestingly, this article says there is no law in Florida requiring the station to conspicuously post prices.

As it happens, I was back in Orlando today and noticed that the station, which is now called Suncoast Energy, has increased its location premium from about 30% (roughly $0.85 above market of ~$2.85 in my OP) to more than 115%!! Today, they were charging $4.99 for regular, $5.09 for mid-grade, and $5.19 for premium. The stations a mile north on Semoran were charging $2.29 for regular.

At those prices, it actually would be cheaper to let Hertz fuel the car at $2.30 per gallon plus $6.99 service charge (which is their current offer for refueling service) unless you'd used less than 3 gallons of gasoline.

I recall that when I originally posted, I was outraged at the station. The fact that they are still in business and have almost quadrupled the premium they are charging says to me that either:
-People really do value convenience when they're returning their rental car and are willing to pay absurd amounts for it, in which case I'd grudgingly concede that the station has a viable business model; or,
-Enough people are either not very observant or bad at math that it doesn't matter what the station charges, which is kind of pathetic.

Sadly, I suspect it's the latter.