the promo code that is being used is stored in a session variable already.
If you followed the instructions from the previous posts, the session will be named PromoCode.
In the store order summary behavior, you should bind the promo Code column to the PromoCode session variable.
you may need to add the following code at line 1 of the confirm page to force php to "see" the session:
<?php if(!session_id()) session_start(); ?>