vb2008 HttpWebRequest "post"

[andyc2002]

Hi all, I have problem on use HttpWebRequest with "POST" method.
First, I access
http://store.apple.com/hk-zh/configu...=UNLO CKED%2FWW
this link and go to the cart page.
Then when I change the quantity to 2, I had see the live http header to use "POST" and submit the content to the server.
I follow the header and write with vb2008, but I get the server return 503 error on the code that getting response, would you mind to tell me that I got problem on where?

logincookie = New CookieContainer()
     Try
     'access item page
     req = DirectCast(WebRequest.Create("http://store.apple.com/hk-zh/configure/MD198ZP/A?option.iphone_applecare=none&option.iphone_bumpers=none&option.iphone_4_dock=none&option.additional_apple_usb_power_adaptor=none&option.additional_apple_dock_connector_to_usb_cable=none&add-to-cart=add-to-cart&cppart=UNLOCKED%2FWW"), HttpWebRequest)
     req.Method = "GET"
     req.UserAgent = "Mozilla/5.0 (Windows NT 5.1; rv:9.0.1) Gecko/20100101 Firefox/9.0.1"
     req.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
     req.Headers("Accept-Language") = "zh-tw,en-us;q=0.7,en;q=0.3"
     'req.Headers("Accept-Encoding") = "gzip, deflate"
     req.Headers("Accept-Charset") = "UTF-8,*"
     req.KeepAlive = True
     req.CookieContainer = logincookie

response = DirectCast(req.GetResponse(), HttpWebResponse)
     For Each tempCookies In response.Cookies
     logincookie.Add(tempCookies)
     Next

     reqreader = New StreamReader(response.GetResponseStream())
     htmltext = reqreader.ReadToEnd
     reqreader.Close()
     response.Close()

than post data(i sure that the post content are correct)

 byteData = encoding.GetBytes(postdata)
     req.ContentLength = byteData.Length
     postreqstream = req.GetRequestStream()
     postreqstream.Write(byteData, 0, byteData.Length)
     postreqstream.Close()

'server return 503 in this code
     response = DirectCast(req.GetResponse(), HttpWebResponse)

     reqreader = New StreamReader(response.GetResponseStream())
     htmltext = reqreader.ReadToEnd
     reqreader.Close()
     response.Close()

i have error on "response = DirectCast(req. GetResponse(), HttpWebResponse )"

For programmers: provide error codes, snippets of code related to error code(no longer than 20 lines)
sorry i may be more than 20 lines a little bit...