https Error on checkout.php

I have a branded public store at http://stlamhhof.com/store/ I just had a call from a user who said when they placed something in the shopping cart and clicked the checkout button, the received the attached warning that they were going to a site that is not private and attackers might be trying to steal their information. . When the user starts on the site, they are entering under http. When they click on the "checkout" button, then are sent to https. I am using the standard Power Store checkout.php page. This checkout page allows them to create a login or login if they are a returning customer. On top of the checkout page, I have the script:

<?php if ($_SERVER['SERVER_PORT']==443) header("Location: http://" . $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF']); ?>

Not everyone is having this issue. Do you have a suggestion on how I can fix this?

You have a script referenced in your footer:
<script src="http://crm.stlamhhof.com/webTracker.php"></script>

since that script is referenced from an insecure location it will alert the user that there are insecure portions of the page.