It is my understanding that anyone can ship a gun to an FFL anywhere in the USA. Non-FFL's cannot use the USPS; FFL's can. As far as the GCA 1968 is concerned, Puerto Rico is just like any other state.
(2) The term "interstate or foreign commerce" includes commerce between any place in a State and any place outside of that State, or within any possession of the United States (not including the Canal Zone) or the District of Columbia, but such term does not include commerce between places within the same State but through any place outside of that State. The term "State" includes the District of Columbia, the Commonwealth of Puerto Rico, and the possessions of the United States (not including the Canal Zone).
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Puerto Rico, State
Regulation: (2) 478.11