I have lived in Florida for about 20 years, on and off. Here is what you need to know about SunPass when you rent a car:
FL does NOT have many toll roads, the turn pike (easily avoidable), some roads around Orlando, Miami and in Tampa Bay, most of which offer the option to pay your tolls in cash. However, toll booth have been eliminated on part of those roads, leaving the driver only two options: SunPass or TollByPlate.
SunPass itself will NOT do so, they will just charge the regular toll. The rental car company will charge you an "administrative fee" if they receive a toll invoice from SunPass, basically making additional money from your rental.
Before renting a transponder from the car rental company for a lot of money per day, check where you are going on maps.google.com - this will give you a good idea if there are toll roads on your intended route and if you can avoid those. Google Maps allows you to route your trips avoiding toll roads ;-), which in FL is pretty easy, unless you are in a real hurry.
I usually check rental car prices via priceline.com as I rent cars on average at least one/week. You can find a good post on how to do that at: ninjatips.com/how-to-rent-a-car/
Sorry for the screwed up links, but because I am new here, I can't post links... (copy and paste will work though!) If you still have question about how to use priceline or renting a car in FL, feel free to send me a message
PS: If you do have a transponder and happen to drive through a cash lane, no worries your transponder will register and the attendant will know you paid - they won't charge you twice ;-)