HttpWebRequest add item to cart then "post" to change quantity problem

[andyc2002]

Hi all, I have problem on use HttpWebRequest with "POST" method.
First, I access
http://store.apple.com/hk-zh/configu...=UNLOCKED%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?

Here is my code:

Dim logincookie As CookieContainer
        Dim response As HttpWebResponse
        Dim tempCookies As New Cookie
        Dim req As HttpWebRequest
        Dim postdata As String
        Dim encoding As New UTF8Encoding
        Dim byteData As Byte()
        Dim quant1 As String
        Dim quant2 As String
        Dim quantint As Integer
        Dim totalquan As Integer
        Dim cartid As String
        Dim quant3 As String
        Dim htmltext As String
        Dim cartid2 As String
        Dim reqreader As StreamReader
        Dim postreqstream As Stream


        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()


            'use "POST" to change the quantity to 2
            req = DirectCast(WebRequest.Create("http://store.apple.com/hk-zh/hybrid_cartx"), HttpWebRequest)
            req.Method = "POST"
            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"
            req.Headers("Accept-Charset") = "UTF-8,*"
            req.KeepAlive = True
            req.Headers("X-Requested-With") = "XMLHttpRequest"
            req.ContentType = "application/x-www-form-urlencoded; charset=UTF-8"
            req.Referer = "http://store.apple.com/hk-zh/cart"
            req.CookieContainer = logincookie
            req.Headers("Pragma") = "no-cache"
            req.Headers("Cache-Control") = "no-cache"



            'construct POST data and write to stream
            quant1 = "maxlength=\""3\"" id=\"""
            quant2 = "quantity\"" /> <\/li> <li class=\"
            quantint = InStr(htmltext, quant1) + quant1.Length - 1
            cartid = htmltext.Substring(quantint, InStr(htmltext, quant2) - 1 - quantint)
            totalquan = CInt(TextBox1.Text)
            postdata = cartid & "quantity=" & totalquan.ToString
            postdata += "&" & cartid & "packageText=&" & cartid & "messageText=&"
            quant3 = "_a=up&cartid="
            cartid2 = htmltext.Substring(InStr(htmltext, quant3) - 1, 100)
            postdata += cartid2.Substring(0, InStr(cartid2, Chr(34)) - 1)
            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()

            WebBrowser1.DocumentText = htmltext

        Catch ex As Exception
            MsgBox(ex.ToString & vbCrLf & ex.Message.ToString)
        End Try

I had comment req.Headers("Accept-Encoding") = "gzip, deflate" because when I set the encoding to server, the response are encoded and I don't know how to decode...
Thanks a lot