Tracopa Bus, Calles 14-16, Ave 5.
Their terminal is not in the best part of town so grab a taxi to get there or walk down mid- morning to get your ticket. The terminal is well posted and has clean washrooms. Buses leave promptly ontime. Checked baggage is ticketed and you must produce your stub to retrieve it. I travelled to Golfito via the new western motorway, and we made two rest stops enroute which was a relief. Their buses are a little older than some of the other lines but were clean and reasonably comfortable.