You specified the page when you implemented authorize.net, so yes it does have to do with that.
the thank you page I believe is for after you register... or maybe after you send an email contact.
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.