The store order summary server behavior is applied to the confirm.php page.
open the confirm.php page.
In the server behaviors list, double click the Store Order Summary server behavior.
you dont need to change anything on the first step of the Store Order Summary Server Behavior, so click next to get to the second step.
On the second step, select the coupon column from the list of columns that are in the order table, and click the lightning bolt next to value.
Expand the Session Collection and select the Coupon session variable.