In this scenario you need to set it up in your Bill of Materials.
The sales orders will be recorded against the individual item's SKU, but the supplier orders will be made for the box SKU.
So, since the individual item is the sold product, this will be the FINISHED PRODUCT in the BOM.
Then the box will be the COMPONENT in the BOM.
The quantity will then be the inverse of the box quantity (ie: 1 divided by the quantity).
So for example if you sell chocolate bars (with a sku code of CHOC), and order them in a box of 10 (box Sku code CHOCBOX10), your BOM will be:
Product: CHOC; Component: CHOCBOX10; Quantity: 0.1
Since the sale of a single bar is essentially 0.1 (or 1/10th) of a box.