Okay, I'll try it. I thought if you added it to the top of the success page it would destroy the cart data before it is posted ..?
When I put it before the closing body tag I get this error:
Warning: Cannot modify header information - headers already sent by (output started at /usr/www/users/lhunter/checkout_success.php:21) in /usr/www/users/lhunter/checkout_success.php on line 311