The ixmlserializable type 'system.xml.linq.xelement' cannot be deserialized in partial trust because it does not have a public parameterless constructor.

Rather than have your CreateCreditCardCustomerCode function return XElement, have it create a custom class (that you will need to write) with the properties it needs, populate the properties from the XElement and return the custom class instead. And please don't ask how to do that or ask for the code as it should be obvious that you have no provided anywhere near enough detail for anyone to give you specific advice.