Unfortunately, promotional code/coupons can't be added after an order has been placed or applied to a previous purchase. In order to receive the discount, promo codes/coupons need to be added to the cart page or during the checkout process. Please refer to How do I apply a promotional code? and How do I use the coupon?
If you forget to add a promotional discount code, you can still make use of this code on any new orders for as long as the promotion is valid.
Please contact our Customer Service team if you have any questions regarding your promo code.