Tip: To restock faster, you can punch the slab shop while holding the item it trades. Crouching and punching at the same time will empty your inventory from that item and stock it into the shop quickly.
You may also choose to right-click a chest with a stick. Through that, you'll be able to dump all your stock in the menu that opens up, and that will automatically stock your shops within a 32-block radius with the appropriate items.
Finally, if you want to delete the slab shop, you can do /ushop destroy. All the items stocked within it will be dropped as soon as you destroy the shop. The slab itself will not be removed either.