Make sure you've already set up your shipping discount preferences in My eBay.

To make the item you are listing eligible for the discount, select the "Apply a shipping discount when a buyer purchases multiple items from me" check box. 

If you have multiple profiles for your combined shipping discount rules, select the profile name to apply to the listing.

A message appears in your listing showing the availability of a discount if the buyer purchases other items from you.

If the buyer commits to buy another item and makes a combined payment, eBay automatically calculates the shipping discount and displays it to the buyer.