try putting the
<?php
if (isset($_POST['Submit'])) {
$_SESSION['location'] = $_POST['location'];
}
?>
block towards the top of the page, before the //WA eCart Redirect Check Out block.
I suspect when you are submitting the form, you are being redirected to the checkout page before the session is set
(are you setting the session on submitting the 'checkout' form ?)