Hertz cars that participate in the program actually have the SunPass transponder unit installed on inside of the windshield. It's in a little metal box. If you want to use the transponder you open the "door" on the transponder so that it registers at each toll plaza you travel through.
Many rental companies instead participate in a Toll-by-Plate program. In those cases you don't carry a transponder but instead your rental car's tag/license plate is photographed at each toll plaza as though you were a toll violator. The toll authorities match these up to the owner (the rental car company) and bills them and they subsequently bill your credit card.
Either way, keep in mind that as you travel to Disney, to the space coast or anywhere in Florida, you stay in the lanes marked SunPass or E-PASS and pass by at highway speeds. You bypass the toll booths.
BobK/Orlando