I've only generally hired with Alamo & there is no transponder in their cars - the licence plate is just enabled for SunPass. There is a daily charge - but I am 99.9% it's only levied for the days you drive through the SunPass tolls (tolls obviously charged in addition to the daily charge Alamo make). There is, I believe a daily cap on the tolls, so if you plan a long trip down to, say Miami, you will be quids (or rather dollars)-in as you will hit the cap quite easily.
No idea what happens if you try to register an Alamo licence plate (already registered with SunPass) onto another SunPass account. In theory the "computer should say no" - worst case scenario is the OP's daughter could be charged on both her Fathers Sunpass account & also by Alamo, or best case & the licence plate being on 2 accounts = no charges!
On my last few Alamo rentals, the agent has explained it quite simply with no hard-sell. Told them the only tolls we would incur on during the holiday would be from/to the Airport on the SR528 & we were just told "use the cash lanes, it's cheaper" (true).
About 5 years ago I hired from Avis & their car did have a SunPass tag glued to the windscreen, but it had an on/off lever on it. Am sure I was told to keep the lever "off" to avoid the daily charge if I just wanted to pay cash (which I did).
Slightly OT, but shame the system is not as simple as in France. Have got a house there & our family have one Telepeage (toll tag) between us. It matters not what vehicle uses it (UK registered / French rental car etc) - you can lend it to anyone, but it's always the registered owners bank account that gets the hit for the tolls incurred. You can also use the tag for quite a lot of city / town centre carparks too.
