public function shippingMethod(array $values)
    {
        $existing = array_filter($values, function ($method_id) {
            return (bool) $this->shipping->get($method_id);
        });

        if (count($values) != count($existing)) {
            return $this->translation->text('@name is unavailable', array(
                '@name' => $this->translation->text('Shipping')));
        }

        return true;
    }