add_action('template_redirect', function() {
  if (is_page('208')) {
    $referer = [home_url('/sanpai/'), 'https://buy.stripe.com/test_aEU3eR1af5EA6Q07ss'];
    if (!isset($_SERVER['HTTP_REFERER']) || !in_array($_SERVER['HTTP_REFERER'], $referer)) {
      wp_safe_redirect($referer[0]);
      exit();
    }
  }
});