What you need to do is go to Seller Hub.
Click on the "Site Preferences" link.
Click on the "Business Policies" link.
This is where you create the combined shipping policies you want to use. You may have one policy for all of your listings or multiple policies for different listing.
Once you have created the different policies you want to use you will need to update each one of your listings with the policy you want to assign that item.
Once this is done Ebay will automatically combine the shipping once the buyer puts the items in their shopping cart.
