public function checkoutService($price, $currency, $locale, $country = null)
    {
        $this->_checkAmount($price);

        $params = array(
            'merchant_id' => $this->config['eid'],
            'total_price' => $price,
            'currency' => strtoupper($currency),
            'locale' => strtolower($locale)
        );

        if ($country !== null) {
            $params['country'] = $country;
        }

        return $this->createTransport()->send(
            new CheckoutServiceRequest($this->config, $params)
        );
    }