get value return in google checkout

I am working with google checkout and currently working at sandbox environment. I have used Paypal and i thought google checkout should work like the same. but i wonder that google checkout is not posting back the HTML form field on return URL and im not able to find any proper solution for this problem yet. Here is my code.

here im using a custom field shopping-cart.merchant-private-data to get the data but i dont know how to collect that data on continue URL.

I just need to pass some ID in a custom field so that on return URL I can get it back and then change my database on the basis of this ID. There is one thing which is important that Upon return google checkout is destroying sessions also and there is no value stored in sessions. This made a 开发者_StackOverflowbig problem for me. So please tell me how i can get that custom ID back from google checkout.

Use the Notification API to get transaction information back from Google Checkout.

http://code.google.com/apis/checkout/developer/Google_Checkout_XML_API_Notification_API.html

When you get back the notification, you can read your custom ID from the merchant-private-data field.

Also see this answer which touches on a related issue.