Unfortunately, we can not insert the coupon on an active order due to privacy/customer protection reasons and systems limitations.
However, you're still good to use your coupon in your next order, as long as you use it before its expiry date.
To use it, simply paste it in the coupon code box, right before finalizing the order.
The coupon box is located below the total amount of your purchase at the checkout page and in your shopping bag. Make sure that it doesn't have any extra characters or any blank spaces when trying to redeem it.