A discount code expires once it used. If you get a message saying the code you entered is invalid, it could be due to various reasons including an unsuccessful payment. In this case, please contact us at firstname.lastname@example.org so that we can re-issue a valid code for your use.
For certain promotions, the discount code would only work for specific items under a specific category. Please do read through the fine-print for the promotion to understand how the code would work.