One cause might be that the system is if you're using a saved address on file for your billing information that has outdated information in it. Double check your Address Book by visiting http://www.rifftrax.com/user and clicking on the "Billing info" tab. There you'll find all of your saved addresses used for shipping and billing purposes. Make sure these addresses (especially the one set as your "Default") has the correct information.
If all of these addresses are correct (or if you're entering a new address/zip at checkout) and you're still getting this error, try contacting your bank, and ask them to check on this payment attempt. It's possible that their information is either incorrect, the charge attempt is being erroneously flagged, or there may in fact be an issue on our end that we're not aware of. If that's the case then please submit a Help ticket and we'll investigate.