so, that error happens if you have links to resources that use fully qualified URLS using the http:// protocol.
on your checkout page, there are links to blog pages: pages:
<li class="profile firstlink"><a href="http://blog.enhancedhealing.com/" target="_blank" class="ps_buttons ">Blog</a></li>
these links are causing the problem. change that to use the https:// protocol instead:
<li class="profile firstlink"><a href="https://blog.enhancedhealing.com/" target="_blank" class="ps_buttons ">Blog</a></li>
you have a few other links as well that use the http:// protocol, and at the end of the page, you include a script:
<script type="text/javascript" src="http://webplayer.yahooapis.com/player-beta.js"></script>
this will need to be edited too.