Hilton Head isn't really known for its shells--but the common ones are
whelks, cockles, augers, some wentletraps, and other common shells of the
area. There is a guy, Ron Elliott, who will take you out at Edisto, to two
of the close-in barrier islands.  Shelling is pretty good there, but
nothing unusual.
 
I think I have Ron Elliott's phone number if they want it--let me know.
 
Carol