In Sosua, you can check out this website from Banker Trust.
Mail Service in the Dominican Republic
Also, there is a post office in Puerto Plata, it is near the central park. Last year I sent a box (30 pounds) from the US and a friend had to wait a month and also had to pay 1500 pesos to receive the package. One thing for sure though, it is best to have the name and phone number attached to the shipping label. I think they now have a website for mail service in DR.
Thanks DRExplorer! I have checked out the website!