Do you have a link for your site, I’ll recomend using .eq for the query. Your database will have the title promo code which can be like, "backToSchool and then another column with the discount, another one could be valid true/false. When you make the query It’ll return the object containing the information where you can extracted from so you can tell the user that the code has expired, is not valid or the actual discount value.
When presenting it in text it must be a string and a number for operations.