Hi Joy,
In order to use eBay's shipping calculator when creating your listings through Auctiva, you just need to check the bubble next to "Calculated Shipping" in the "Shipping Tools" section of the Auctiva lister page. Then, enter your the weight, size, and dimensions of your package in the approrpriate fields and select the shipping services you'd like to offer from the provided pull-down menus.
Ebay's shipping calculator will then be displayed on your listing and your buyers will be able to get quotes for all of the shipping services you are offering by entering their postal codes into the calculator. Also, during the checkout process, the buyer will have the option to choose any of the shipping services you specified as available when creating the listing.
Any additional questions or concerns? Please file a support request using the appropriate link on our help page:
http://www.auctiva.com/free-auction-software-help/help.aspx-Mike