<merchant-calculation-callback />
is used in Merchant Calculations API - its a callback in the Checkout process (aka Part 1 in the Google Checkout API)- Google sends the callback to your systems asking for calculations - e.g. shipping, tax, promotions, hence the name merchant-calculation-callback.
Part 1/Checkout - is the part where you send data to Google for Checkout. Therefore it is not going to give you information related to "after completing payment"
Part 2/Processing Checkout Orders covers the APIs you can use to obtain order data "after completing payment" - primarily the Notification API. These APIs are what you must implement in order to get data for integration with your own order-processing systems (e.g. order status, payment status, saving order data, updating inventory, etc.).
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