Google Checkout Accessing Merchant Calculations Callback

I'm implementing the Google Checkout service into my web service using the XML api. I have the cart request and all of that setup, and am using the Merchant Calculations api for shipping (My service does shipping calculations)

I'm having difficulties figuring out how to access the response that is sent to my server with the buyer's address / other personal data for the order and determining their shipping request.

Is being sent in the header? as a POSTS value? GET? How do I access it.

I'm developing using PHP

The merchant calculations request is in XML format in the body of an HTTP POST.

Some useful links below:

Format of the Merchant Calculations request / response: https://developers.google.com/checkout/developer/Google_Checkout_XML_API#merchant_calculations_api

https://developers.google.com/checkout/developer/Google_Checkout_XML_API_Merchant_Calculations_API#Handling_Merchant_Calculation_Callbacks

How to configure the web service to respond to callbacks: http://support.google.com/checkout/sell/bin/answer.py?hl=en-GB&answer=70647

PHP library - look at responsehandlerdemo.php which shows how to parse and respond to a merchant calculations request:

http://code.google.com/p/google-checkout-php-sample-code/