Park staff may be appreciative of an offer to help them find out what lives there, especially if you are willing to help develop a collection for them.  Bureaucrats are less likely to be flexible, and customs agents are unpredictable.  The only trouble I have personally encountered with customs was in trying to return specimens borrowed from the Royal Tyrrel Museum in Alberta.  I think the customs agent, upon hearing the word "fossil", envisioned some spectacular dinosaur despite the box being under 25 cm max.  We ended up leaving the box with Customs and contacting the museum to let them negotiate its release.  Perhaps if we had gotten out the specimens and showed them to the agent, it would have disillusioned her as to the value of the package.  Paleozoic clams, cut into pieces in order to study the shell structure, do not have high market value.
Leaving the Philippines, my brother was questioned by a customs agent as to what was in all the containers.  She insisted on looking into each one but showed no further interest in each as they turned out to contain shells.
He also discovered a useful technique at airports.  His backpack was full of fossiliferous rocks and thus largely opaque to the X-ray machine.  "What's in that backpack?"  "Well, first there is a nice piece of Ordovician limestone.  Note all the bryozoans and brachiopods, and a nice bivalve in the corner..."  "OK, you can go."
The Department of Agriculture is suspicious of land snails as potential pests, so "seashells" is better than "snails".

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!