The code that is used on your confirm page to process the transaction does not even take into account the credit card type. It just takes the number, date, and card code. So there is nothing that I can tell that the code is doing to cause this to fail.
In the full response that you posted here is the message you are getting from Authorize.net:
The configuration with processor is invalid. Call Merchant Service Provider
The error seems to be clearly telling you that it is a configuration issue with your account and that you need to contact them. I will gladly do whatever I can to help you get this worked out but it seems that the issue is with the configuration for the provider. I hate to just point back to Authorize.net but everything that I'm seeing here, including the messaging directly from them indicates that the issue is on there end and not with the script that is processing the transaction. Maybe there is a check box or another agreement that has not been confirmed to enable processing of Mastercard.
