Coming back from Cortona with a rental car to be returned in Rome will require first that you refuel the car.
This is the refueling station on the freeway I saved in my favorites when I return my car in Rome downtown (save it on your google maps):
Service Area "La Rustica Nord"
Once you refuel the car, the Termini garage where rental cars are returned is the one located under the station on Via Marsala.
It is called Parcheggio Piastra
This is the itinerary from the gas station to the return garage, however depending on the traffic Google Maps may send you through alternate itineraries. It doesn't matter as all those roads lead to Rome (the Termini station parking) and none of them go through a ZTL.
If you want you can ask the car rental desk to call you a taxi. Alternatively you can walk to the front of the station where the taxi rank is, where dozens of taxicabs are lined up, as suggested by Christine (this is the cheapest option because if you call a taxi the meter starts at the moment the taxi is dispatched to you). The taxi rank in front of the station is here on Piazzale dei Cinquecento. It is a little over 1/4 of a mile walk from the Parking garage where you return the car.