Hi,
This method works for USPS and UPS, as they are supported for Calculated shipping with eBay's shipping calculator.
Create Combined shipping rules in your eBay preferences and sync them to Auctiva in your Auctiva account settings for eBay. Then select Calculated Shipping and check-off "Apply my eBay combined shipping rule" on the Auctive one-page-lister for your listings. Make sure you have one or more common shipping methods selected for the listings, e.g. USPS Priority, or the transactions won't combine.
Danno