I am using Version 188.8.131.52 of the wp e-commerce plugin, with wpec-authorize-sim plugin to connect to my authorize.net account. If I check the "Relay Response (similar to IPN)" checkbox in the Authorize.net SIM gateway settings section in the Payment tab of the store settings, my checkout page cannot even connect to the authorize.net payment page, telling me that the relay response url doesn't match. I entered my checkout page url to the Relay Response URL section in my Authorize.net account, and I also tried my transaction results url, but they were both not accepted.
What is the relay response url for the wp e-commerce plugin? I would think it is either my transaction results url, or the checkout page url, but neither of them worked for me. So it must be something else. What should it be? O,r how can I find out?