protected function setExpressCheckoutRecurringPaymentConfig($data, $subscription = false)
    {
        $billingType = $this->billingType;

        // Overwrite billing type if $subscription flag is enabled
        if ($subscription) {
            $billingType = 'RecurringPayments';
        }

        // Send L_BILLINGTYPE0 and L_BILLINGAGREEMENTDESCRIPTION0 only if there is billing type
        if (isset($billingType)) {
            $this->post = $this->post->merge([
                'L_BILLINGTYPE0'                 => $billingType,
                'L_BILLINGAGREEMENTDESCRIPTION0' => !empty($data['subscription_desc']) ?
                    $data['subscription_desc'] : $data['invoice_description'],
            ]);
        }
    }