where you are passing a querystring variable to tell the server what page to load, you will need to modify the forms action to also pass any querystring variables that are passed in:
<form name="PayPal_ExpressCheckout_form" action="<?php echo $_SERVER["PHP_SELF"]; ?><?php echo (isset($_SERVER["QUERY_STRING"]) && $_SERVER["QUERY_STRING"] != "")?"?".$_SERVER["QUERY_STRING"]:""; ?>" method="post">