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

To make the item you are now listing eligible for the discount, select the check box "Apply a postage discount when a buyer purchases more than one item from me".

When a buyer commits to buy one of your items, a message appears 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 postage discount and deducts it from the total cost of the purchase.