In that plaza across the street from Playero.
No,the post office is not across from Playero. To get to the post office, you go past Playero, past the Texaco station to the plaza on your left if going toward Cabarete. More or less across the street is a small supermarket. It used to be called Productos Sosua, but now, as I recall, it has a different name. But you will have no trouble recognizing it as a supermarket.
The store in the plaza where the PO is that is closest to the highway is a well-marked paint store. The post office is at the back end of the plaza.