In the orders table, the applied coupons ID is stored in the orderCoupon column.
you would need to modify the recrdset on the orders-details.php page to return the coupon information:
LEFT OUTER JOIN ps4_users ON OrderUserID = UserID
INNER JOIN ps4_coupons ON couponCode = CouponID
WHERE OrderID = ParamOrderID OR ( -1= ParamOrderID2 AND OrderID= ParamSessionOrderID)
you could the show the copon code vale in the prder information.
the entered coupon code is stored in a session variable, you coulfd display that on the cart page:
<?php if(isset($_SESSION['WA_ECart_Coupons'])) echo $_SESSION['WA_ECart_Coupons']; ?>