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I'm working on shipping rate calculator going back and forth via XML. The
thing I'm confused about is that the code works, but after a few hours and I
don't know how many requests, I start getting the error "The underlying
connection was closed: Could not establish trust relationship for the
SSL/TLS secure channel." I can't imagine UPS's certificate is bad.

Here's my code, let me know if you see any glaring issues.

HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create(upsRateUri);
myReq.Headers.Clear();
myReq.Method = "POST";
myReq.ContentType = "application/x-www-form-urlencoded";
myReq.ContentLength = xmlString.Length;
StreamWriter sendingData = new StreamWriter(myReq.GetRequestStream());
sendingData.Write(xmlString);
sendingData.Close();
WebResponse myResponse = myReq.GetResponse();