http://wiki.civicrm.org/confluence/display/CRMDOC/Google+Checkout+Configuration

To setup Google Checkout Merchant-Accounts you can use the following links:
TEST Merchant Account : http://sandbox.google.com/checkout/sell
LIVE Merchant Account :  http://checkout.google.com/sell

Google Checkout does not allow you to use the same account as both the customer and the merchant for a transaction. Therefore you will need to create both a Merchant Account and a Customer Account for testing purposes. You can create your TEST Customer Account(s) at http://sandbox.google.com/checkout .

Adhere to the following guidelines when setting up your test accounts:

•  Skip any sections that ask for your bank account information.
• Enter any name and address as long as the State field contains a valid two-letter abbreviation for a U.S. state and the Zip Code field contains a five-digit or nine-digit zip code. (You do not need to enter the correct zip code for the address.)
• Enter any 10-digit phone number for the Phone Number field.
• Use one of the credit card numbers in the following table:

  Card Type	Card Number	CVC	Expiration Date
  American Express	3782 8224 6310 005	any four digits	any four digits
  Discover	6011 1111 1111 1117	any four digits	any four digits
  MasterCard	5555 5555 5555 4444	any four digits	any four digits
  VISA	4111 1111 1111 1111	any four digits	any four digits

Configure API Access

Set API callback URL

1. Login through your Google Checkout Merchant-Account.
2. Click on 'Settings' tab.
3. Go to 'Integration' section.
4. Set 'API callback URL' to http://path-to-civicrm/extern/googleNotify.php
5. Also make a note of Merchant-ID and Merchant-Key displayed under 'Account information'.

Set Merchant-Key

1. Go to 'Payment Server Settings' in civicrm.settings.php file.
2. Set 'CIVICRM_CONTRIBUTE_PAYMENT_TEST_KEY' ( OR CIVICRM_CONTRIBUTE_PAYMENT_KEY if LIVE ) to Merchant-Key obtained from step 5 of  'Set API callback URL'.

Set Merchant-ID

1. Log into CiviCRM as an administrator.
2. Select Administer CiviCRM.
3. Select Global Settings.
4. Select Payment Processor.
5. For Payment Processor, choose 'Google Checkout'.
6. Set the 'Merchant ID' to Merchant-ID obtained from step 5 of 'Set API callback URL'.

Set Checkout URL

• For TEST transaction, set 'Checkout URL - TEST' to 'sandbox.google.com/checkout'.
• For LIVE transaction, set 'Checkout URL - LIVE' to 'checkout.google.com'.

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Regards,
Munish Kalra