Add product fields to cart form through line items
To be absolutely clear, you want the "Quantity Per Box" field present on product entities to display on the cart view?  For that you'll need to add I believe the "Commerce Line Item: Referenced Products" relationship in the view.  That should give you access to the product fields, with no need to copy a field from product entities to line item entities.

Since the cart is a Commerce Order view:
Relationships needed to reach products can be visualized as:
commerce_order->commerce_line_items->commerce_product