ServicePointManager.ServerCertificateValidationCallback = CheckValidationResult;
var cert = new X509Certificate2("cert.crt"); // Используем чтобы проверить подлинность удалённого сервера  
byte[] data = ....;  
byte[] resp = PostForm("https://xml.foo.com/api", data, new X509Certificate2Collection() { cert });

public bool CheckValidationResult(object sender, X509Certificate certification, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
      return true;
}

protected byte[] PostForm(String url, byte[] data, X509Certificate2Collection authCerts)
        {
            try
            {
                var req = WebRequest.Create(url) as HttpWebRequest;
                req.ClientCertificates.AddRange(authCerts);
                req.ContentType = ContentTypeEnum.PureForm.ToString();
                req.Method = "POST";
                req.ContentLength = data.Length;

                using (var os = req.GetRequestStream())
                    os.Write(data, 0, data.Length);

                var resp = req.GetResponse() as HttpWebResponse;
                if (resp.StatusCode != HttpStatusCode.OK) throw new WebException("ошибка в ответе веб-сервиса");

                var respData = new byte[resp.ContentLength];
                using (var rs = resp.GetResponseStream())
                    rs.Read(respData, 0, (int)resp.ContentLength);

                return respData;
            }
            catch (Exception ex)
            {
                throw new Exception("Error", ex);
            }
        }