private function setHeaders()
    {
        $headers = [
            'X-PAYPAL-SECURITY-USERID'      => $this->config['username'],
            'X-PAYPAL-SECURITY-PASSWORD'    => $this->config['password'],
            'X-PAYPAL-SECURITY-SIGNATURE'   => $this->config['signature'],
            'X-PAYPAL-REQUEST-DATA-FORMAT'  => 'JSON',
            'X-PAYPAL-RESPONSE-DATA-FORMAT' => 'JSON',
            'X-PAYPAL-APPLICATION-ID'       => $this->config['app_id'],
        ];

        return $headers;
    }