No, you would never want to carry an envelope or package through customs (or, for that matter, through airport security) without knowing the contents.

But as to Cuba, you are allowed to bring certain gifts. (I think it's a 1000 Peso maximum)

Take a look at this site: http://www.cubaabsolutely.com/Travel/Useful-information-&-FAQs-Cuba.php?id=Customs-regulations

It's Canadian so it's not completely applicable to the U.S.  But you might find some worthwhile into in there.



RHaj