It's been a couple of years since I used it, but I have a Sunpass sticker for driving a rental car in the Miami, Fort Lauderdale, Keys area. I bought it online, it was mailed to me, registered it and preloaded a small amount ($10) of prepaid funds to take the tolls from. Instead of peeling back the sticky, the phone agent suggested I use Scotch tape to tape it to the windshield and keep the rental car transponder closed at all times. The stickers are one-time use, so using Scotch tape allows for multiple uses. Before I leave the rental car parking garage, I call the Sunpass office or use the app to give them the license plate number. You'll have to give them an end date and time to prevent unauthorized use. She also suggested taking the sticker off every night because it's attractive to thieves and they WILL break into your rental car for your transponder.
For two weeks of driving around south Florida, I paid about $20 in tolls. It's well worth it.