In the event that you attempt to pay for a letter(s) and you receive an error code, it could mean that your credit card was declined. If you have extra security features placed on your card it could also cause issues with submitting payment.
Additionally, we have seen payment issues come up when incorrect information is entered, such as the card number or zip code. Double check the information and make sure everything was entered properly.
If you are still experiencing payment issues, we might be able to give you a general reason as to why what the issue is, but we unfortunately don't have any detailed insight into this so we might recommend reaching out directly to your bank first in order to find out exactly what the issue is.