MarketPress Checkout not working with Authorize.net

Hello,
I believe I'm on the home stretch here with only this issue and a smaller one that is still pending. I do believe that everything is configured correctly (Store, Cart and Checkout pages) as well all of the necessary requirements met (Gateway acct, SSL cert etc.) with information inserted into the Payments form for Authorize.net (Login ID, Trans Key etc.) I've gone over it a dozen times on both ends (MarketPress and Authorize.net acct) fixing all of the errors and issues and now I'm down to this. When I try to process a credit card sale (with a current, valid card) I get this error message:

[Oops! An error occurred while processing your payment.

There was a problem finalizing your purchase. The credit card has expired. Please go back and try again.]
I'm not using an expired card and I've also tried a few other cards with the same result. From what I can see the transaction is never making its way to the Authorize.net Gateway so Auth.net is not sending this error message.

The tool for you to access the website is open so any suggestions will be greatly appreciated! MarketPress is wonderful and I'm very impressed with it as well excited and anxious to release the website so I hope you can assist. Thank you!

Regards,

Mark