EZBuyButton Buy Book Widget for Authors plugin allows you to embed the EZBuyButton on Posts, Pages, and on the Sidebar. EZBuyButton is a "buy now
Here are four screenshots of our plugin. The first screenshot is of the EZBuyButton Settings page. The second and third screenshot displays the EZBuyButton Embed Code custom field when you're on the edit/add new mode of Posts and Pages. The fourth screenshot is of the EZBuyButton Embed Code Widget that you can add to the sidebar.