Some of the confusion among the posted answers cause me to add this prefatory note: There is one province called Newfoundland and Labrador. There is no province called Newfoundland and there is no province called Labrador.
"Newfoundland and Labrador is" or "Newfoundland and Labrador are"?
I think I would write "Newfoundland and Labrador is the only province in which license plates are pink with purple polka-dots" (except that they're not, AFAIK, but that's a different topic).
If I were going on vacation and intended to visit both Newfoundland and Labrador I might write "Newfoundland and Labrador are where I'm going", and if I were going to Newfoundland but not to Labrador I certainly wouldn't write that, nor "Newfoundland and Labrador is...." etc.
In this matter, are there standard conventions adhered to by all sober literate people and blessed by learned authorities and taught to children in all schools?
What other polities called "A and B", where A and B are two disjoint geographic regions? (I seem to vaguely recall that "Serbia and Montenegro" was once a thing. Not sure I'd count "Budapest".) What do they do with "is" and "are" in other such cases?