Using Sun Pass (Prepaid Toll Program) (Orlando)

Hi everyone, we're staying in Orlando for 3 months and are wondering what the best/cheapest way is to pay Toll for the FLorida Highways.
Usually we use cash, but now that we're staying for a longer period we think using the Sun pass scanner is cheaper.
The 'problem' is that our rental company Budget charges $4 p/d to use it...
Does anyone know if there a cheaper way to use this system?
Where can I buy it, and it that recommended?