We manufacture a product AAA and pack it in a box (we are using BoM for its packing material). We are selling it in pcs and using  multiple big packs e.g Box of 12 PCS, Carton of 4 Boxes (48 Pcs of the AAA.) to pack it. We are buying Box($1) and Carton($2) from a Vendor. Now if we sell 48 PCS of AAA, we will pack it in 4 Boxes (12 Pcs of each product in 1 box) and pack these 4 boxes in Carton. (we are not charging packing cost to our customer). Now my question is how can we control packaging material's inventory and how could we reflect its cost in expenses. I did not find a way to control inventory and account for the packaging material other than MRP. We could not use MRP in this case because in MRP you have to produce a new product (not our case). 

Any help or guideline will be greatly appreciated.