The key thing with embedded, plugin-enabled content is to set the embedded object as "transparent", so that the HTML-based cart can "show through" the plugin content. In your case, you might need to consult Google Earth documentation or developer support to ask how to set their content as transparent in your page, so that a "lightbox-style" overlay (such as our cart) can show through the Google Earth content.
The test URL you linked does not appear to be available anymore, but I was able to put your sample Google Earth code on a test page of my own, and the cart appeared mostly fine there; except for an odd bit of the Google Earth content showing through the cart's header bar (where the cart's "Close" link normally appears), the rest of the cart showed up fine for me.
