public function requestSetExpressCheckout($amount, $returnUrl, $cancelUrl, array $optionalParameters = array())
    {
        return $this->sendApiRequest(array_merge($optionalParameters, array(
            'METHOD' => 'SetExpressCheckout',
            'PAYMENTREQUEST_0_AMT' => $this->convertAmountToPaypalFormat($amount),
            'RETURNURL' => $returnUrl,
            'CANCELURL' => $cancelUrl,
        )));
    }