On the checkout page, create the checkbox, make sure to give it a checked value:
<input name="local" type="checkbox" id="local" value="1">
On the confirm page, create a hidden form field with the same name, Set its value to come from the checkbox on the checkout page:
<input name="local" type="hidden" id="local" value="<?php echo((isset($_POST["local"]))?$_POST["local"]:"") ?>">
Then add a set session value server behavior. Set the trigger to any form post and set it to get the value from the checkbox:
<?php
if (!session_id()) session_start();
if($_SERVER["REQUEST_METHOD"] == "POST") {
$_SESSION["local"] = "".((isset($_POST["local"]))?$_POST["local"]:"") ."";
}
?>