Nora;
        Notice you didn't get a major response re Cuba.  The USA has almost no
truck with this last remnant of the so-called Evil Empire (ie,
falsely-called communist countries affiliated with the defunct USSR).
Canada does, but for shells it would not do much good anyway, since the
export of natural objects such as shells is strictly verboten - i have
heard that this regulation is not strictly enforced, but still, if your
take is inspected by Canadian official who are on the ball, you will
lose them, and perhaps more, if the officiant is having a bad day!  The
mail system, foreign currency regulations, etc., etc., make getting
shells out of the country by postal purveyance a rather uncertain
proposition - so i think it is best all around to abandon thoughts of
bringing molluscan exoskeletons and other such remnants of
Cuban-dwelling non-human species, except from dealers who have access to
old material.
In the Great Wet North,
Ross.