public string GetJson()
{
var json = string.Format("\"monto\":{0},", Currency);
if (TransactionId != ulong.MinValue)
json += string.Format("\"trx_id\":{0},", TransactionId);
if (PaymentMethod.HasValue)
json += string.Format("\"medio_pago\":\"{0}\"", ((int) PaymentMethod.Value).ToString("000", CultureInfo.InvariantCulture));
return "{" + json + "}";
}