Originally Posted by
Out of my Element
Not sure why you would question that. There is never customs or immigration going to and from domestic destinations, and Puerto Rico is as domestic as Washington, DC is.
But a separate agricultural regulation zone so there is an extra check for flights leaving PR, just like leaving Hawaii. Yes, it is a domestic flight but that doesn't mean it is necessarily 100% identical to other trips. In this case it should be, but there are plenty of little quirks in the way the world works. :-: