I'm selling product by the case which has a field on the Product Display called unit quantity which determines how many units there are in each case. I'd like to display this information on the order line item using a custom field on the Product line item type.

This field value should be automatically copied into the line item field when a product is added to the cart.

I've looked at the the walkthrough video for custom line items types however the usecase describes adding the custom field to the cart for the user to fill out (i.e. registration name).