We are unable to retroactively add a coupon or gift card to an existing order since the payment is already completed. If your client didn’t apply a discount code at checkout, you can choose to honor the amount they would have saved, and process a partial refund. You will need to do this through your payment processor (Stripe, PayPal) directly.
Learn more: Refunding an order
Tip: If you choose to honor a discount retroactively, you may like to deactivate the coupon or gift card to prevent it from being used at a later date. In your Coupons or Gift Cards page, simply click ••• > Disable.