You can pick up one of the thin, adhesive SunPass toll tags at any number of retailers across Florida (Publix, drug stores, etc.) for $4.99. Despite all the documentation they provide saying it only works if you peel off the film and adhere it to the glass such that the glass works to conduct/amplify the signal (??? glass is an insulator...), it works perfectly fine if you firmly hold it up to the window as you pass under the toll gantry. Two well-placed pieces of Scotch tape work well, too. It then conveniently fits in a pocket or your wallet or whatever when you're done with the rental.
If you log in and tie your rental car's license plate to the account and the transponder does fail to read, then (unlike with E-ZPass) it will correctly associate the rental car's plate with your account (not the rental car company's) and will charge you the TTOL (transponder toll) rate rather than the ITOL (image toll) rate, at least on MCO-area toll roads.
You can also pay $20 for the plastic, portable, E-ZPass-style transponder, but I see no need to spend the extra money and carry along the extra bulk. I also see zero reason to pay the rental car companies to use their service when you can buy your own in three minutes for only a pinch more than paying the rental company for one day of a toll pass.
Source: three trips to Florida in the past year.