Enterprise Solutions: Manufacturing and Wholesale desktop user.

We offer free shipping on a few select products. How can we track the free shipping cost within the sale? We would like to track the free shipping as a Discount on the Sales by Item Summary report. Is this possible at all, let alone as a Discount? It just doesn't seem possible to add this onto the invoice as it would lower the grand total the customer paid us. I've tried by adding the shipping cost we paid to the sale as a line item then taking the free shipping "Discount" that I set up but that messes up the report by showing more shipping cost income. Hoping someone can help. Thank you.