Most likely, the success page is missing the require_once line to attach the cart include file, but I cant say for sure without seeing the code.
make sure code similar to the following is at the top of your page:
//WA eCart Include
Where <Cart Name> is the name of your eCart object.
To clear a session variable add :
<?php if($_SESSION['variable_name']) unset($_SESSION['variable_name']); ?>