travelling from Rome to Sorrento round trip

If you arrive at FCO at the right time, you can take a regional train to Tiburtina and a Marozzi bus from there directly to Sorrento. Buses depart at 07:00 and 15:00 Monday-Saturday and at 07:00 on Sundays. Or you can take a Curreri bus directly from Tiburtina to Sorrento. Departures are at 14:00 Monday-Saturday. No service on Sunday. For Sorrento-Rome the Marozzi bus departs at 06:00 and 17:00 Monday-Saturday and at 17:00 on Sunday. The Curreri bus departs at 06:30 Monday-Saturday. No service on Sunday. For the most flexibility with lots of departure times, take the Leonardo Express train to Termini, a EuroStar Italia high-speed train to Naples, and a Circumvesuviana commuter train from Naples to Sorrento. Do the reverse when you head back to FCO.

Thank you, that is what I figured. We don't arrive into Rome until 8:45 and we want to leave Sorrento around 1pm. That leaves out the 4 hr. bus. I will try to find a good transfer company and maybe I can get a transfer from FCO to our hotel in Sorrento for a reasonable rate.

Do you mean 8.45am? If so, how about a fast train to Naples and a transfer from Naples train station to your hotel in Sorrento (75 to 90 euro)? Have done that and was straightforward.