Items and cases

Hello
I'm trying to solve a way to keep my inventory updated.
I sell icepops. I make them at my factory, and make x pcs cases.
Them I sel either cases or pcs.

I've tried to make the cases kits, but when I sell them (wholesale) it's a little problematic for my customers.
I've tried to "manufacture" the cases, but when I make an invoice, my inventory is not affected. The same happens when I sell pops individually.
I send X number of cases to the store, and then, I sell the pieces that make that case.
But the inventory is not affected.

Has anyone had this kind of problem? And how did you solved it?
Thanks in advance.