public PaymentUpdateResponse PaymentUpdate(PaymentUpdateRequest request)
        {
            if (request == null)
                throw new ArgumentNullException("request");

            return SendPostRequest<PaymentUpdateRequest, PaymentUpdateResponse>(request, "PaymentUpdate");
        }
        public PaymentUpdateResponse PaymentUpdate(string merchantTransactionID, PaymentStatusCode paymentStatusCode)
        {
            if (merchantTransactionID == null)
                throw new ArgumentNullException("merchantTransactionID");

            if (paymentStatusCode == null)
                throw new ArgumentNullException("paymentStatusCode");

            var request = new PaymentUpdateRequest
            {
                MerchantTransactionID = merchantTransactionID,
                PaymentStatusCode = paymentStatusCode
            };
            return PaymentUpdate(request);
        }