Just wanted to post a tip that worked for us in FL.

You can take off that sunpass transponder on the windshield (ours was velcroed on) and go into any CVS (or other places listed on sunpass.com) and buy the sunpass sticker for $5. Ask the cashier for a couple of pieces of scotch tape and stick those temporarily on the plastic box the sticker comes in.

Use your smartphone (or whatever) to create an account at sunpass.com and register your sticker. There is an option for registering your rental car. Choose that option and input the license plate and the duration of your rental. The sunpass sticker will come preloaded with the $5 you already paid. I added an extra ten dollars (the min. reload amount). It says it takes a couple of hours to be totally in the system.... ours seemed to be activated immediately.

Use the scotch tape to tape the sticker to the windshield. I know the instructions say it needs to be stuck to the glass - but you don't need to remove the plastic backing to make it work. Just use the scotch tape.

Now drive through the sunpass lanes easy peasy and don't worry about the awful fees Avis charges for using their toll system. You can even logon to your sunpass account and see that it is working and the tolls you were charged.

The roads to the orlando airport don't say Sunpass but they work with the sunpass system.

This worked for us. I found this tip buried in a Disney board somewhere and it has worked for others. Peel your sticker with the tape off before you return the car (but after going through all the toll roads!). Save it for your next trip! I have also read that if you are not planning to return, you can call and have your remaining balance credited back to your card. I have not tried this yet but since I have like $8.75 left I might...

I hope this helps someone else avoid the fees. Happy Travels!