It worked for me and took me to the success page (please refund my payment).
I think the issue is probably that you started with:
http://midhudsonweb.com/mhwdev/reyousables/post-details.php?id=3 (without the www.)
and it redirects you to the www. version of the site, which has a different session. The solution is to use .htaccess to force the user to always use the www. version of the site.