your checkout button is a standard submit button. your trigger code:
if (isset($_POST['eCartMGP_Checkout_x'])) {
will only work for image element types. for a standard submit button, it needs to be:
if (isset($_POST['eCartMGP_Checkout'])) {
you may also want to add @session_commit();
<?php
if (isset($_POST['eCartMGP_Checkout'])) {
$_SESSION['location'] = $_POST['location'];
@session_commit();
}
?>