I inquired what time the express buses to Santo Domingo leave the Metro bus station in Sosua last week, and I was told that there are none. The girl working behind the counter said that all the buses run the same route stopping in Puerto Plata and Santiago.
So I took a so-called express bus on Caribe Tours. The only difference between the express buses and their regular buses is that the express bus bypasses Santiago. I took the express bus from Sosua to Santo Domingo, and it took 4 2/3 hours. The following day I returned to Sosua on a regular non-express bus, and it took 5 1/4 hours. Thus the express bus only saved about 35 minutes.