Clear plastic (polystyrene) boxes should not deteriorate and attack
the finish on shells, but what you put in the bixes (what you mount
the shells on inside the boxes) can be a problem.  Avoid any kind of
"foam rubber" or plastic foam products.  Even though such products
today are more stable than they were 20 years ago, some of them can
still decompose, releasing acids that can dull the gloss on a cowrie
or olive shell.  In old collections I have purchased, some foam
inserts have deteriorated to the point where they were gummy, or else
hard and brittle.  And, in one old collection , many of the shells
mounted on this kind of material were badly damaged.  In fact, some
of the thinner specimens, like Epitoniidae, actually had holes eaten
through the underside of the shell, where it was in contact with the
foam.
Personally, I like surgical cotton, which I buy in 1-pound rolls.
However, some folks distrust even cotton, and use some kind of glass
wool or fiberfill.
Paul M.