this code:
<?php if ((isset($_POST["ReceiveEmailSpecials"]))) { echo "<strong>Yes, I would like to receive specials via Email</strong>"; }?>
is not referencing a Session variable, it is referencing a POST variable, which is created when the checkout form posts to the confirm page.
post information is only available while the forms action page is loading.
in other words:
The action of the form on the checkout page is to post to the confirm page.
the POST variables only exist while the confirm page is loading, once it is finished loading, the POST is cleared.