salerno-amalfi coast

I don't know of passes, unless the Campania Unica card works for your itinerary. But either the ferry or bus is easy. There may be a small discount for roundtrip, but then you are committed to a specific ferry.

SITA bus, pick it up either adjacent to the Salerno Centrale train station, or along the main drag in Salerno. Tickets are dirt cheap, 2-3 Euro, bought at the station or any tabacchi shop. It is 1 hour to Amalfi. To Positano you change buses in Amalfi, then another hour. Fun ride, great scenery, make sure you sit on the driver side outbound, passenger side on the way back.

We very much preferred the ferry to the bus, but there are a number of smaller towns along the coast where the ferries don't stop due to the lack of a dock. If you want to stop anywhere other than Amalfi-town or Positano, the bus is a better option.