Then put up signs at beaches saying "Taking sand is illegal unless you do it in a government vial". Allow tourist sellers to resell the vials on the beaches.

Then at airports and ports, allow those vials to go out filled with sand. Ban the import of vials (probably no need to enforce, because filling tiny containers is unlikely worth it on a criminal scale).

It's a very low effort way to tax a product without requiring tax returns and paperwork.