Can't Get AuthorizeNet SIM Relay URL Working!

[function]

basically, we are having the customer go through the checkout process via FirstData e4 Global Gateway, put in their Credit Card info, and then we want to show them the Successful Check Out Confirmation Page once finished. However, the line: 'MODULE_PAYMENT_AUTHORIZENET_TEXT_ERROR_MESSAGE' in authorizenet.php keeps getting triggered. We know this because the error message is popping up even though there is no problem - and the payment is being processed.

Been on and off the phone with Gateway Support at First Data e4. We think the relay URL is wrong. The Relay URL that we are putting in is:

http://domain.com/cart/index.php?mai...eckout_payment

via "Link to the receipt page including result parameters using HTTP POST method (POST)" (on their settings)

Brings us back to Step 2 of 3 on the Zen Cart Check Out (Payment Information Page).

I tried to put in:

http://domain.com/cart/index.php?mai...t_confirmation

http://domain.com/cart/index.php?mai...eckout_success

And both did not work! Still takes me to Step 2 of 3 on the Zen Cart Check Out (Payment Information Page).

Please help!! Client is angry lol