Species protection and borders
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Endangered species
Some shells, corals and other natural material are from protected species. Where that is so, possession, sale and movement are restricted independently of where the item was found.
International listings
CITES governs international trade in listed species, including a number of corals and molluscs. A souvenir brought home from a holiday can be exactly what it covers, and the enforcement point is the border.
Which catches ordinary tourists
Giant clam shells, queen conch and many corals are the common examples. The purchase in a market abroad is not what makes it lawful to import.
Occupied shells are a separate question
Several states and many parks prohibit collecting a shell with a living animal in it, regardless of species. That is a conservation rule and it is widely posted and widely ignored.
Coral is usually a no
Both as a species matter and under park rules. A piece of coral on a beach is one of the finds most likely to carry a restriction.
Driftwood and sand sound harmless
And are still regulated in many places, with quantity limits on sand and outright prohibitions in national parks. The harmlessness is about the object rather than the rule.
Keep the provenance for these especially
Where, when, and under what permission. For anything that might later need explaining at a border or to a buyer, the record made at the time is the only one worth having.
This property cannot answer it for your find
Species identification plus jurisdiction is a specific question. What it can do is make sure you know the question exists before the shell is in a suitcase.
Customs is where it is noticed
Not at the beach. A find that travelled home in a bag is examined when it crosses a border, which is also the point at which the paperwork you do not have becomes relevant.