Session variables won't be available on ipn pages. The IPN page is posted to by the Paypal server, so it won't have any session variables from the user available. You need to use the values posted from paypal on the ipn 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.