public function setExpressCheckoutOptions($credentials)
    {
        // Setting API Endpoints
        if ($this->mode === 'sandbox') {
            $this->config['api_url'] = !empty($this->config['secret']) ?
                'https://api-3t.sandbox.paypal.com/nvp' : 'https://api.sandbox.paypal.com/nvp';

            $this->config['gateway_url'] = 'https://www.sandbox.paypal.com';
            $this->config['ipn_url'] = 'https://ipnpb.sandbox.paypal.com/cgi-bin/webscr';
        } else {
            $this->config['api_url'] = !empty($this->config['secret']) ?
                'https://api-3t.paypal.com/nvp' : 'https://api.paypal.com/nvp';

            $this->config['gateway_url'] = 'https://www.paypal.com';
            $this->config['ipn_url'] = 'https://ipnpb.paypal.com/cgi-bin/webscr';
        }

        // Adding params outside sandbox / live array
        $this->config['payment_action'] = $credentials['payment_action'];
        $this->config['notify_url'] = $credentials['notify_url'];
        $this->config['locale'] = $credentials['locale'];
    }