How to add an item that is the same as another, so that when sold, they both reduce the quantity of one item?
Question might be vague, but here's an example of what I'm trying to accomplish:
Let's say the item is a box of pens,
You can buy a pen separately, or you can buy a box of pens (contains 4 pens). I can't seem to figure out how to make an item where if you do buy a box of pens or just one pen, it would reduce the parent item which is one pen. So that if you buy a box of pen, that would reduce the parent pen item by 4 pens.