You can buy insurance for international 1st Class shipments by purchasing the insurance through Auctiva. Check out this link
http://www.auctiva.com/help/pages/My%20Account/Checkout...ettings/Overview.htmIf you make insurance optional in your listing, make sure that you have the "use Auctiva Shipping Insurance" and "Auto-purchase Insurance" options selected on your Auctiva account settings--that way, if your buyer wants insurance, it will automatically be purchased through Auctiva.
Hope that helps a bit