PayPal IPN Response

Question

I am having a problem with PayPal IPN response. After payment is made by the customer, PayPal IPN returns this URL:

www.mywebsite.com?orderid=32&tx=2AC67201DL3533325&st=Pending&amt=2.50&cc=USD&cm=&item_number=32

There are a couple of issues

1. Post-back field names are undefined or missing. Thus I can get the INVALID message. I am not sure if my website does not read POST variables. When I looked at IPN history, it shows that each IPN has been sent with the complete url.
2. Payment status keeps coming Pending. Does this issue cause the first issue?

Thank you for your responses in advance.

Here is the code:

    Dim strSandbox As String, strLive As String
    Dim req As HttpWebRequest
    strSandbox = "http://www.sandbox.paypal.com/cgi-bin/webscr/"
    strLive = "https://www.paypal.com/cgi-bin/webscr"
    req = CType(WebRequest.Create(strSandbox), HttpWebRequest)

    'Set values for the request back

    req.Method = "POST"
    req.ContentType = "application/x-www-form-urlencoded"
    Dim param() As Byte
    param = Request.BinaryRead(HttpContext.Current.Request.ContentLength)
    Dim strRequest As String
    strRequest = Encoding.ASCII.GetString(param)
    strRequest = strRequest & "&cmd=_notify-validate"

    req.ContentLength = strRequest.Length
    'Response.Write(strRequest)
    'Send the request to PayPal and get the response
    Dim streamOut As StreamWriter
    streamOut = New StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII)
    streamOut.Write(strRequest)
    streamOut.Close()
    Dim streamIn As StreamReader
    streamIn = New StreamReader(req.GetResponse().GetResponseStream())
    Dim strResponse As String
    strResponse = streamIn.ReadToEnd()
    Response.Write(strResponse)
    streamIn.Close()
    If (strResponse = "VERIFIED") Then

        Response.Redirect("thankyou.aspx")
    ElseIf (strResponse = "INVALID") Then


    End If