First, the Dominican postal service is very unreliable (corrupt? shady?) so if you sent something from the United States chances are it may get "lost".
I have an acquaintance in the DR and he has a Miami-based courier service for any US-based mail to make it to him in Santo Domingo. He doesn't trust his postal service. Before he got the courier, he strongly recommended using a 3rd party shipper like UPS, FedEx...but if you've ever checked those prices it's prohibitive.
But then again, if you're sending a five-cent card to Stan Javier in the Dominican and you just want to see what happens, you could try an International Reply Coupon which works like return postage on a SASE. I've never tried it myself, but you could read about it here: http://en.wikipedia.org/wiki/International_reply_coupon