Alamo/National/Enterprise use the toll by plate system. There is no turning it on or off, if you use it you get charged, if you don't use it, no charge. The max you would pay for your 3 week rental is 6.00 plus your tolls. If you want to buy a sunpass you can do it via sunpass.com Should you happen to use your own, the Toll by plate system would not interfere or charge you if you have the transponder in place. Personally, I would use the Toll by plate, could be less expensive in the long run. For a description go to:
https://www.sunpass.com/rentalcar Alamo's "rules" are at the bottom.