public string GetOrderUpdateRequest(int orderID, bool invoiced)
{
    XDocument x = new System.Xml.Linq.XDocument(
       new XDeclaration("1.0", "utf-8", "yes"),
       new XProcessingInstruction("webcretexml", "version=\"1.0\""),
       new XElement("WebcreteXML",
           new XElement("WebcreteXMLMsgsRq",
               new XElement("OrderUpdateRq",
                    new XElement("OrderUpdate",
                        new XElement("ID", orderID),
                        new XElement("Invoiced", invoiced))))));

    return x.ToString();
}