If the mini cart module is on the same page as the buynow module then no additional configuration is required. If you add the mini cart to a page that does not have the buynow module on it then you must add the following line in your web.config:
<add key="MiniCartSetting" value="Home"/>
Replace “Home” with the page name that has the buynow module. The page name property can be found in the page settings.