Regional-Metrpolitan trains

Paestum and "Pompei" both have stops on the national Trenitalia railway system, so that's an alternative versus going to Naples and catching the Circumvesuviana commuter train to "Pompeii Scavi" station. Looking at schedules through DB both legs of the trip are called regional, with a connection at Salerno. This Pompei station is closer to the Anfiteatro entrance to the ruins; but since you're traveling with luggage, whichever entrance you leave your luggage at is the one that you have to return to. This strategy was discussed on your earlier post. See also a map of the grounds.

The train schedule from Paestum is regional to Salerno. The train from there is called Metropolitano. Not the Circumvesuvius train. They are listed together on the Trenitalia site schedule.I assume one ticket works for this trip?

William,

If you decide to travel via both Trenitalia and Circumvesuviana on that trip, you'll need two separate tickets as they're different rail networks.

On trenitalia's site if you purchase a ticket from A (Paestum) to C (Pompeii) via B (?), you can go from A to C via B. If it doesn't sell the last stretch, for whatever reason, you get a ticket from A to B and you're supposed to purchase the last ticket in Italy. For Regionale trains there is a 7 days limit to buy tickets on-line.
Maybe you already know it, but if you're going to visit the archaeological area in Pompeii note that everybody uses the Circumvesuviana for a good reason: their station is almost in front of the main entrance.

EDIT: I checked on trenitalia. They sell both the tickets. It's on their site: from Paestum to Pompeii via Salerno it costs 4.2 Euro one way per person. Why did you doubted it?

I was not familiar with the Metropolitano train. I just wanted to make sure it was at the same station in Salerno. The station at Paestum has limited service and I just wanted to be prepared. We are staying at a B/B between both stations in Pompeii so no need to go to Naples and down. Thanks for your help.