I do this:
Buy a box from Office Depot. In the 21 1/2 x 15 x 12, you can get two stacks of slabs side by side.
buy some packing peanuts and bubble wrap
put a piece of cardboard on the bottom
put a layer of peanuts on top of this
I wrap five slabs together at a time by folding each into bubble wrap. I fold cardboard in on the 4th and 5th slab, creating a cardboard shell. This has one layer of bubble wrap between each slab, then cardboard on the outside, then a final wrap of bubble wrap.
Seat the bundle on the peanuts. Then I put a piece of cardboard on top of that. Then another bundle. Remember, I have two stacks going, side by side.
Then, I put a sheet of cardboard on each end of the box and between the two stacks. These are upright to provide structural support against crush damage.
I fill in with peanuts as I go.
Add bundles as you can, but leave 3-4 inches at the top for peanuts, then bubble wrap (big bubbles here if I have any laying around), then a cardboard piece.
then plenty of heavy duty shipping tape
I have sent loads of slabs to ComicLink this way without problem via USPS