When I went to Jamaica for the 1st time I sent a few of my family and friends postcards while I was there.
Postcards are really cheap. I think you can get 3 or 4 for $1. And you can buy them anywhere, the airport, the resort, off the resort at the local vendors, etc. I'm not sure how many you are planning on buying, but remember, you can always negotiate prices (usually off the resort).
The only thing is, people didnt get the postcards for a while (like 2 to 4 months later!) Also, I think stamps were like 60 cents each, if I'm not mistaken. So that's something to take into account.
With all that said, I love the idea! If I do it, I think I will have them fill it out and give it to us while we are still there.