Most of the shells you find washed up on the beach are dead; of course, I always throw the live ones, which are very rare to find, back into the sea. I'd rinse them in fresh water and then leave them outside before you pack them; that reduces the odor of any organic material that may be left in the shell. Then repeat the process in some water and Chlorox when you return to the States. After they've dried, use a little clear nail polish to coat them on the outside. Makes any colors look bright. There are no regulations against collecting shells, but corals are another matter. The Belize government doesn't allow you to take corals, whether they were collected dead or alive, back home. Since they can't determine whether they were collected alive, and would rather not have people taking "souvenirs" from live reefs, they made a blanket rule that no corals can be taken. But, you know rules...