Hi rugriok,
I'm sorry to hear about the difficult experience you have had. Ebay does not allow items that use different checkout systems to be combined during the checkout process so, unfortunately, what you have described here is expected behavior when a buyer purchases multiple items that don't all use the same checkout system.
Since you disabled the "Use Auctiva Secure Checkout" setting in your account, any listings you post through Auctiva thereafter will be processed through the eBay Checkout system, but the listings that you already posted through Auctiva while the Auctiva Checkout setting was enabled will continue to be processed using Auctiva Checkout.
There is not a facility available, either through eBay or Auctiva, to remove the third party checkout configuration from am active listings that is already set up to use third party checkout. Therefore, the only way to change the checkout system used by your Active Listings that are currently set up to use Auctiva Checkout would be to end them early and repost them with the Auctiva Checkout setting disabled.
I hope this helps. If you need any further assistance with this, please feel free to contact our customer support team using the form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx#
-Mike