The Logged into users access should be using the UserID session variable not the checkout querystring.
Is that error coming from the Gateway or the Shipping provider, that info should be on the confirm page.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.