Can I get FTP access? I'll have to play with the code to debug it and see the include files. Running the live page doesn't do me much good.
I don't see a problem in the code on the page you sent, so perhaps the problem is in one of the includes.
I'd start by dumping the session on the success page and see if it looks like it thinks it sent correctly:
<?php
@session_start();
var_dump($_SESSION);
?>