Discount codes cannot be combined. If a customer has a 10% off discount, they can't also use a "free shipping" coupon. Only one discount at a time can apply to a single order.
Please follow these steps if the customer is not able to use a discount code created through the Returns app.
From your Shopify admin, click Discounts.
Click the name of the discount that you want to view the details for.
Check if any rules are set for the discount codes.
If there is an expiry date set for a discount, then the discount will expire at 11:59:59 PM on that day.
If the discount code is applied to specific variants of a product, then the customer can use the discount code for that specific product.