Like Chirimoya says, Caribetours and Terra buses go there. I did see a Metro bus at the border, but I don't think they have a regular service there and it was a chartered bus or something.
Caribetours is 1300 pesos + $25 (border formalities) SD-PaP and $40 + $28 PaP-SD (obviously other visa costs if you're a Haitian or Dominican citizen or resident). Buying a return ticket is probably a bit more economical. Buses leave SD every day at 11am, arriving in P?tionville at about 5.30pm, and everyday at 8.30am from P?tionville, arriving in SD at about 5pm.
Food is served on the bus. The seats are a lot more spacious than the regular Caribetours buses. Tickets are sold at the travel agency office at the entrance of Caribetours in Santo Domingo. if you want to now more, just send a pm.