With paypal it should be storing the OrderID on pp_confirm.php page not the success page. Make sure the session variable used to store the OrderID is the same as the one displayed on the success 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.