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When checkout out with a Diners Card you get the error: "You have entered an invalid card security code.". This is because we validate only against the first digit of the card. American Express cards begin with a 3 and have a 4 digit CVV but Diners also start with 3 and have a 3 digit CVV.
Diner cards are always 14 digits long where as American Express are always 15. I've added a check against the length when the card starts with a 3 for now. Obviously some more robust credit card checking could be added in the future.
I would be happy to take this on if nobody else has already started.