Kiln shelves are heavy, brittle, and chip at the edges, and a cracked shelf can fail mid-firing. Pack them on edge with padding between each one, and treat them like large ceramic plates.
Pro tip: Pack shelves on edge with a rigid divider between each, exactly like transporting glass or slabs.
Watch out: Never lay kiln shelves flat and stack them; the weight and any flex crack them, and a hairline crack can fail during firing.