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).

I've also thought about adding a relationship to the order line item view to link back to the product display (through product) and simply display the field however i fear that this will pull back the live data for historical records.

Is there a way i can do this with Rules? something i'm missing?