Allow the user to combine different "brands' into a single item in the inventory
Suppliers like to help their customers, and if they don't have exactly what was ordered, they'll send a substitute item. So for example, we might order 3 x 23 litre kegs of IPA (total 69L), but the supplier might send 1 x 20L, 1 x 23L, and 1x 30L. So now I've got 3 different items in my inventory, and a total of 73L of IPA.
If I create a new item for each keg, then each one will have a par. If I set par to zero for the "wrong sized" kegs, the inventory is going to carry on trying to order more IPA based on the fact that it only has 1 x 23L.