Personally, I would use ALL 2 letter or ALL 3 letter abbreviations.

When I think of the Dominican Replubic I think of "DR"...not "DOM".

But with two letter abbreviations, you might run into some problems.

One suggestion is to use all 3 letter abbreviations, and simply add an "X" on the end, if you need to.

So..Dominican Republic would be "DRX"..just my .02..