You have to edit https.php. There is an array called $https_scripts. The scripts listed in this array will always be available over HTTPS.
Add
Code:
$https_scripts[] = array("help.php?section=contactus");
It should look something like this when you are finished
Code:
$https_scripts[] = "secure_login.php";
if ($config["General"]["use_https_login"] == "Y") {
$https_scripts[] = "register.php";
$https_scripts[] = array("cart.php", "mode=checkout");
$https_scripts[] = array("cart.php", "mode=auth");
$https_scripts[] = array("help.php?section=contactus");
}