IME, returning rental cars in Italy, more than a few times with Hertz, is a very fast and simple affair: Let the agent or attendant know the car is there and go! It's an unsettling experience due to its speed and ease compared to returning a car in North America. Never had an extra charge other than what was on the invoice at checkout.

As I mentioned, Termini is fine if you research where the location is before hand. I hadn't fully (or the location hasn't been explicitly defined in other threads) which is why my pin (red box) was in the wrong place in the 1st map. I'd return there again if I was leaving by train which is what I did.

I suppose if you wanted to really make it easier, you could return at CIA (don't forget to check out the newly-opened palatial ruins in the Via Appia Antica on the other side of the GRA while out there) or FCO and hire a transfer services mentioned in other threads here.