- You recently moved, and changed your address at the bank, and
- You are attempting to checkout with a saved card on file.
This is a known issue that currently exists due to the way we save your card in our secure servers (using Stripe). Basically when you saved that card the first time, it saved it with the billing address you used then. This means if you moved, when we try to charge your card on file, it is attempting to also validate that old and out-of-date billing address.
SOLUTION: Delete the card or enter it fresh.
Right now there is no way to update your address in the Edit form (and our system overrides any address entered into the "billing info" field at Checkout if you use a saved card). Until this fix is in place, the easiest thing to do is to either enter the card as if it were a new one (using your chosen address) OR go into your My Account area, click the "Saved credit cards" tab, and then delete it from the system. Then you can re-enter it at Checkout with the updated address intact.