Sosua Post Office
I would love to say that this exists, but dream on, Pollyanna.
Yes. There is a "post office" in the same off-the-street plaza as where you go to pay Edenorte. There is also an "INTERNATIONAL EXPRESS MAIL" box at the photo/cable/viagra store on Pedro Clisante.
From time to time, I amuse myself with sending postcards or letters to friends and family in the US. Just to keep it real, and to make sure it is a real sample, I use various locations, ranging from Florida to Massachusetts to New Hampshire, Connecticut, and California.
The fastest delivery time was a postcard to Connecticut last summer and it took 9 weeks. The rest of my recipients are still waiting. Bastedly, I assure you.
The only way you can count on mail leaving the North Coast is by a.) dropping it when you change planes; b.) giving it to friends to drop when they return; c.) using your EPS or whatever other carrier you use; or d.) cutting off your arm and using UPS, DHL, or FedEx.