For stores using Shopify Markets and Catalogs, if a fee product is not available in all of your markets and catalogs, you will see a message at the checkout that says "Unavailable product. These products will be removed from your cart because they're not available in your country/region".
This often happens for Shopify Plus merchants using Shopify's B2B features. For fees to work correctly on these sites, you need to make sure that all products with the type of “Fee” and “Fee Placeholder” are available in your Catalogs and are available to all of your Markets.