Error on checkout: "This credit card type is not allowed for this payment method. Please enter a valid CVV. Invalid credit card expiration date"

Some users who have added additional extensions or customizations to their store may happen upon one or more of the following errors at checkout, even though the payment extension was setup correctly and a proper card is used.

• This credit card type is not allowed for this payment method.
• Please enter a valid CVV.
• Invalid credit card expiration date.

These errors are thrown by the validation check in our extension, and when triggered erroneously are due to missing data in checkout form submission. Typically this error message implies an issue with JavaScript or alterations to the submitted payment data. The cause is usually site specific and can stem from:

• Various extension installations, including but not limited to one step checkout extensions, third party payment extensions, order comment/attachment extensions, or theme customizations.
• Missing files for our own extension.
• A third party JavaScript file, override to one of our own files, that are acting at checkout.

More often than not this is caused by a third party extension, and the developers will likely have a patch available to resolve this issue.