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