You may me interested to learn that some shells are stopped by customs.
 
Last year I ordered some shells from a dealer in USA to be sent to me in England. At that time I was not aware of the CITES restricted lists, and ordered two clams: Hippopus porcelanus and Tridacna derasa, together with numerous other shells.  The customs in England opened the packages (which said "shells for private study" or similar wording), and impounded these two shells.  The wrote to say I would need an import permit unless the shells were for a recognised research programme or for an educational instititute.  On questioning the English Wildlife authorities they said I would need an export permit from USA.  The US Fish and Wildlife authorities then refused to issue a retrospective permit, and I lost these shells, which were presumably destroyed.
 
The lesson is to find out about from national authorities what is required and what is not. The dealer who sent me these shells told me that he knew export was banned, but that he "had never had any trouble in the past".  He also told me that he could have applied for a prospective export permit (at around $100 per shell), and also that, if detected trying to export shells without a permit, he was in danger of losing his licence to trade.  I was astonished that a dealer would take this risk, and surprised that he did not ask me before mailing the shells whether I wanted to pay for the appropriate permits.
 
I have often received other shells from abroad, and never had any other packet opened as long as the customs declaration says "shells for study"; I guess that I was unlucky on this occasion, but won't take the risk again.  Be prepared for customs officers to open and inspect any package.
 
Tom Walker
Reading, England