I am unsure which transponder you have (there are several models)....the model I have beeps with a green light when the toll is paid (it is battery powered). You can also go online and see the tolls that have been paid (after login)....

https://www.sunpass.com/index

More rental care information:

SunPass Customers:
SunPass customers using a rental vehicle may have their tolls applied to their SunPass account. Customers with SunPass Portable transponders may use their transponders in their rental vehicles. (It is strongly recommended that SunPass customers access their online SunPass accounts or contact the SunPass Customer Service Center to add the rental vehicle subscription to their account for the rental period.) Please remember to remove your transponder upon return of the vehicle.
SunPass customers that do not have their transponders in the vehicle are subject to the tolls, fees and charges set forth below and in their rental agreements.

SunPass Customer Service Center: 1-888-TOLL-FLA (1-888-865-5352)