You will only pay if your customers (or you) buy Auctiva insurance for a shipment.
The rates are here:
http://www.auctiva.com/free_auction_software_help/help.aspx
Look under the "shipping" link.
You can contact customer support here:
http://www.auctiva.com/help/requesthelp.aspx
The link is on the same page that I posted above.
There is no charge for an Auctiva Store. You get it by default.