Advanced Shipping Rates using Box Packing. Flat rate box shipping lets you define costs for boxes to different destinations. Items are packed into boxes based on item size and volume, and then that box uses the fixed costs you define per destination. Add multiple box sizes per shipping zone. | Add costs per box, per weight unit, and a percentage of the item costs inside. | Define a cost for un-packable items e.g. too large to fit in a box.