public function refundTransaction($transaction, $amount = 0.00)
    {
        $this->setRequestData([
            'TRANSACTIONID' => $transaction,
        ]);

        if ($amount > 0) {
            $this->post = $this->post->merge([
                'REFUNDTYPE' => 'Partial',
                'AMT'        => $amount,
            ]);
        }

        return $this->doPayPalRequest('RefundTransaction');
    }