Callbacks is just not working.. and I am stumped as to why. When I enable callbacks, this is the error I am getting after being transferred to the Amazon payment page and entering my Amazon login data:

Either there was a problem connecting to your endpoint or the merchant endpoint returned an invalid response status code. The status code received is 403

In Zen Cart this is the callback URL I am using: https://www.mydomain.com/checkout_by_amazon.php

This is the same URL I have confirgured as the merchant URL in Amazon Seller


Looking at the Amazon error log this is what I see with regards to the callback endpoint

Severity: ERROR
Error Type: OrderCalculationCallbacks
Error Code: HttpError
Error Message: Either there was a problem connecting to your endpoint or the merchant endpoint returned an invalid response status code. The status code received is 403.

Callback Request We Sent:
<OrderCallbackEndpoint>https://www.mydomain.com/checkout_by_amazon.php</OrderCallbackEndpoint>

Cart Request We Received:
<OrderCallbackEndpoint>https://www.mydomain.com/checkout_by_amazon.php</OrderCallbackEndpoint>

I am stumped and quite frustrated..