Discount codes don't stack. If they have 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 Sorted Returns.
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 they have set an expiry date 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.