this code is wrong:
<?php
unset($_SESSION[ssdcart_OrderID]);
unset($_SESSION[PromoCode]);
unset($_SESSION["giftstartDate"]);
unset($_SESSION["giftPrice"]);
unset($_SESSION["giftName"]);
unset($_SESSION["giftCode"]);
?>
you are missing the single quotes around variable names:
unset($_SESSION[ssdcart_OrderID]);
should be:
unset($_SESSION['ssdcart_OrderID']);
or using double quotes in some cases:
unset($_SESSION["giftCode"]);
should be:
unset($_SESSION['giftCode']);
once you fix that, if you continue to have problems, please send a link to the success page.