normally the second line would not be needed if the detail page where set to use a querystring variable.
your detail page is set to use a posted value, so the posted value needs to be included in the add to cart form.
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.