Below is an excerpt from eBay's help article:
"Listings that offer shipping discounts say "Save on shipping." When your shipping discounts are set up and activated, we automatically calculate and deduct them from the total shipping cost. "
ShippingDiscountProfileID: Discounts for different items purchased together. The ShippingDiscountProfileID is used when sellers offer shipping discounts when customers buy multiple items from them.You can find the ID by logging into your eBay account and going to: Home > My eBay > My Account > Preferences > Combined Payments and Shipping Discounts
This is different than ShippingServiceAdditionalCost, which is the cost for additional shipping if more than one of the SAME product is ordered.