Lynnears I agree that you shouldn't comply with that request.  I know why your reacted with no problem to the guy you were sort of caught off guard.  I think it was rather strange a casual acquittance would even ask.  It could very well be nothing but a letter and money but it could be against the law in Cuba.  

I would say that your tour guide company sent out guidelines and rules for the trip.  One was not to carry any unknown packages or the like.