The best way to do this is via a script at the top of the pages you want to ensure are only served up from the https. Here is some code that you can add to the top of these checkout pages that will do this for you:
<?php if((!(isset($_SERVER['HTTPS']))) || ($_SERVER['HTTPS'] == "") || ($_SERVER['HTTPS'] == "off"))header("Location: https://" . $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF']); ?>
This will take them to the same page but with https. Do you know if your secure certificate works with your domain by itself as well as with the domain and www. in front of it? If the certificate only works for www. then you may want to hard code that in the script along with your domain like this:
<?php if((!(isset($_SERVER['HTTPS']))) || ($_SERVER['HTTPS'] == "") || ($_SERVER['HTTPS'] == "off"))header("Location: https://www.<your domain here>" . $_SERVER['PHP_SELF']); ?>