I would suggest setting up the Carton as its own product, priced accordingly for an order of 1 Carton. Then go into Seller Admin > Cart Discounts and create a discount that applies only to the Carton product, taking a flat amount off the per-Carton price when Min Item Qty = 2. Create another such discount where Min Item Qty = 3, and yet another where Min Item Qty = 4.