How can I get an XML response from a request to a remote server?

We are setting up recurring billing on our web app and we are going to be using Cheddar Getter Here's the request dictionary.

We are going to use an existing wrapper written in C# (see it here here)

The problem is, the way the original wrapper was written, we are receiving an http response from the server, and not the xml that the server sends back.

Here is an example method that I've been trying to get to return the xml from Cheddar Getter:

        private static string postRequest(string urlBase, string urlPath, string postParams)
        {
            string result = "";

            try
            {
                HttpWebRequest request = WebRequest.Create(urlBase + urlPath) as HttpWebRequest;

                //Add authentication
                request.Credentials = new NetworkCredential(_Username, _Password);

                //make into a post
                request.ContentType = "application/x-www-form-urlencoded";
                request.Method = "POST";

                byte[] bytes = Encoding.UTF8.GetBytes(postParams);
                request.ContentLength = bytes.Length;

                using (Stream requestStream = request.GetRequestStream())
                {
                    requestStream.Write(bytes, 0, bytes.Length);

                    using (WebResponse response = request.GetResponse())
                    {
                        using (StreamReader reader = new StreamReader(response.GetResponseStream()))
                        {
                            result = reader.ReadToEnd();
                        }
                    }
                }

            }
            catch (Exception ex)
            {
                throw ex;
            }

            return result;
        }

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Using the above method with incomplete data (we're trying to get the gooey details of an error returned from Cheddar Getter), as soon as you call

using (WebResponse response = request.GetResponse())

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a server error is returned (412). This is not desirable, as Cheddar Getter provides XML output with more detailed information about the error (missing name, invalid CC, etc).

If anyone could point me in the right direction, that would be awesome!