public function setBillingType($billingType = 'MerchantInitiatedBilling')
    {
        $allowedBillingTypes = ['MerchantInitiatedBilling', 'MerchantInitiatedBillingSingleAgreement', 'RecurringPayments'];

        if ($billingType != null && !in_array($billingType, $allowedBillingTypes)) {
            throw new \Exception('Billing type is not supported by PayPal.');
        }

        $this->billingType = $billingType;

        return $this;
    }