I have a retail shop, with the product that can be sold as a one piece or as a box. I've created two products with corresponding barcodes and configuration.

• 1 piece has buy - box, sell item. 
• box has buy - box, sell box.
  

All seems good, except I need to keep track of 2 different items in fact I ended up with replenishing boxes, and single item always has negative count.

I could sell 1/6 of a box, or sell 6 items for the 6 pieces box. I've tried but it is complicated for cashier, they always just scan due to time pressure and periodically forget to press the number. I need something elegant yet not complicating cashier life.

How to make it properly? Some sort of manufacturing or something like that?