It seems that whenever I start a project with Zen Cart, it is simple, and then I try to make it complicated and get myself into trouble. That is where I am now.

The short version is that I am attempting to adapt a payment module (Moneris) that is out of date and for the wrong merchant system to work with the payment vendor's current API. I am having a fair amount of success, but I have found that I cannot retain the customer's session when the vendor returns to Zencart. I am sending the session ID in a misc field to the merchant, and they simply echo it back to me, so I have that information in the POST that they are sending. I cannot however figure out how to reintegrate that into the checkout_process.php and would love some help.

Here is the work flow:

[-] Customer chooses payment type for this module.
[-] The form builds on the checkout_confirmation.php page.'
[-] The customer clicks confirm, and the browser is redirected to the merchant's site with the POST info they need to process the payment.
[-] The merchant then redirects the browser back to my cart's checkout_process.php page.
[-] The cart fails out of that page and lands the customer on mainpage=time_out as the session isn't holding through that transfer.

Again, any help you may be able to provide would be awesome!
Thanks!