I have been working with First Data and have come down to this from them:
>You will want to contact your ZenCart support. You are successfully submitting transactions to us, it is just when >we submit the user back to the confirmation page, it doesn’t seem like ZenCart is able to find the variables they >are looking for.
>When contacting their tech support you are going to want to see if there is a way for you to change the URL that >is being submitted through the code. It looks like we’re trying to post your fields to: >http://www.bestconsignmentshopsoftware.com/zen/index.php?main_page=checkout_process and your relay >response is wanting to go to: http://www.bestconsignmentshopsoftware.com
>You’ll also want to double check with them that this is indeed the thank you page that you will want to submit >your payment response information to.
I have searched your code for "&action=confirm", but haven't found a place where I can change it that makes a difference. How can I make the change they need?