lot's of old and partially correct info here ....
I am told there are companies with the 'transponder in a box' still around, but I have not seen this in years. As described on the SUNPASS web site most companies have switched to a fleet 'toll by plate' system so the rental has no transponder. When you pass thru an electronic gate the system first looks for a transponder. If one is not 'seen' the system electronically reads the tag. With a fleet registered car (rental) the system sends the toll bill to a third party company. This is who handles the interface twixt you and the rental company.
If you pay cash, the system has no need to read the tag ....
Most rental companies have gotten very reasonable with their charges to use 'their' electronic toll payment system. Most only charge a $5/day fee, and only if/when you use the system. In other words, if you pass thru an electronic gate you get a $5 fee (plus the toll) and the $5 covers you for 24 hours. previously many companies charged you for everyday you rented the car, charging a daily fee even if you encountered no tolls, and you had to signup when you picked up the car. Not so today - for MOST companies. CHECK THIS WEBSITE::::
https://www.sunpass.com/rentalcar
If you visit Florida regularly and rent cars, and especially if your visits take you to Miami where 95% of the toll roads are electronic payment ONLY, buying a SUNPASS is a good idea. You can buy on the web and have it mailed to you so it is in hand b4 your trip. There is a $5 ONE TIME USE version, and the $20 portable. I strongly suggest the $20 portable as it is intended for use in more than one vehicle, being easy to move about. I've had mine for 10 years and loan it to visitors regularly. There is an app to use to register your rental's tag with SUNPASS which will 'over ride' the rental's 'fleet registration' in case the transponder is misread.
You CAN set up a transponder to AUTO replenish via a credit card, or replenish thru the APP as needed and even replenish with CASH at certain locations including AMSCOT.