According to the Sunpass website
Alamo charges $2/24 hour period plus tolls with a maximum of $6.00 per rental timeframe plus tolls. It also says that participation is automatic (so pass is installed in the car) when you use the Sunpass Only, Express, E-Pass Only or LeeWay Only Lanes. If you decide to go to the toll booth and pay cash you shouldn't be charged. Our daughter just paid the tolls when she rented from them in June and there was no charge for the Sunpass.