Occasionally, your customer may report the following error on the Checkout page.
There are two reasons why this may happen:
1. A security measure to protect your bottom line
Like other 3rd party discount apps, the Discount Mixer dynamically generates a Shopify discount for each order on the cart page. The customer may remove items from the cart after the discount is generated. As a result, they may get a higher discount than expected. Discount Mixer recalculates the discount when items change in the cart, so this is usually not an issue. But it could happen if the customer goes to the checkout page too quickly before the discounts mixer gets a chance to recalculate. So to prevent this situation, the generated Shopify discount is valid only if the cart value was the same or greater when it was generated; otherwise, you will see the error message above.
2. Shopify cache
This is rare, but sometimes Shopify storefront gets into a state for that customer where discounts can’t be created. You can ask your customer to try incognito mode from their browser or clear the caches. Instruction to clear cache is here.