This is the way I THINK the system works - please feel free to correct me:
1. When you go through the EasyPass/SunPass lane, if a SunPass is detected and the car using it has a license plate that has been registered with the SunPass account with sufficient funds to cover the toll, then that account (and only that account is charged).
2. If a license plate is detected that cannot be associated with a particular SunPass account, then the computers attempt to match that plate with a PlatePass account and that account is charged (triggering advertized service fees).
3. If a license plate is detected and there is neither an associated SunPass or PlatePass account, the computer assumes that there was a toll violation and issues a citation accordingly.