Hi Michael,
The first thing you would need to do in order to accomplish this is to set up a new shipping discount profile appropriate for these items in the "Shipping preferences" section of eBay account preferences. Once you have done so, you will then need to update your combined shipping discount profiles within Auctiva by clicking the "eBay Settings" link under the "My Account" tab within your account, and then clicking the "Update Combined Shipping Discounts from eBay" button on the ensuing page.
Once you have done so, assuming you then have multiple shipping discount profiles defined, there will be a drop-down menu next to the "Apply my eBay combined shipping rule" boxes in the "Shipping Tools" section of your Auctiva lister page, from which you can specify which of your combined shipping profiles you would like to use for each listing.
If you have already posted these items to eBay and would like to add a new combined shipping rule to them, you would need to create that combined shipping rule in your eBay preferences and then use eBay's "Revise your Item" form to apply that specific combined shipping rule to the desired listings.
If you have any additional questions or concerns, please feel free to contact our customer support team using the web form on the following page: http://www.auctiva.com/help/requesthelp.aspx
-Mike