How to combine your Google Merchant and Google Checkout Accounts together:
1) Go to google.com/merchants and sign in.
2) a. Go to General.
b. Go to Checkout.
c. If the second option (Sell your content using your Checkout account) is grayed out, then you will click on the Google Checkout hyperlink. If it is not grayed out, choose it and you do not need to complete the remaining steps.
3) Type in your password and sign in to Google Checkout.
4) a. Select "Yes..."
b. Select "Yes..."
c. Enter your password again.
d. Click "Sign in and continue".
5) Fill out the required information.
a. Be sure to have your EIN or Credit Card and Social Security Number ready. If you only use your credit card you will be limited in how much your bank account can receive in payouts per month.
b. Check the box.
c. Click "Complete sign up".
You have now combined your Google Merchant and Google Checkout accounts. You can verify this by repeating steps 1 & 2.