One issue you have is you can access your site from both:
http://preferred-cell-systems.com
and
http://www.preferred-cell-systems.com
That can cause your session to be lost when you switch from one to the other. You can use .htaccess to force one or the other to correct this problem.
I'd need FTP access to fully debug. You could add this to the failure page:
php:
<?php
@session_start();
var_dump($_SESSION);
?>
That would at least show me the error from paypal and might help me give you more information on what might need to be updated.
Did this help? Tips are appreciated...
