Update your checkbox from:
<input type="checkbox" name="red_star" id="red_star" value="" />
to:
<input type="checkbox" name="red_star" id="red_star" value="1" />
Then update your session variable code from:
<?php
@session_start();
if(isset($_POST["Checkout"]) || isset($_POST["Checkout_x"])) {
$_SESSION["red_star"] = "true";
}
?>
to:
<?php
@session_start();
if(isset($_POST["red_star"]) && $_POST["red_star"] == "1") {
$_SESSION["red_star"] = "true";
}
?>
That should do it if your eCart discount rule was also updated and uploaded correctly.