To make it easier for non-profits to receive donations with Checkout, we've launched a Google Checkout donation gadget for Google sites:
It’s easy to add the button right from your Sites page by selecting the “Google Checkout Donate Button” from the Google Sites list of featured gadgets which is accessible through the “Insert” menu and then “More gadgets...”
To learn more about how to create a Google Checkout for Non-Profits account, please visit the instructions page.