' ASP.NET VB

Imports System.Net
Imports System.Net.Mail
Imports System.IO
Imports System.Data
Imports System.Data.SqlClient

Public Class WebForm7
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        'Post back to either sandbox or live
        Dim strSandbox As String = "https://www.sandbox.paypal.com/cgi-bin/webscr"
        Dim strLive As String = "https://www.paypal.com/cgi-bin/webscr"
        Dim req As HttpWebRequest = 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 = Request.BinaryRead(HttpContext.Current.Request.ContentLength)
        Dim strRequest As String = Encoding.ASCII.GetString(Param)
        strRequest = strRequest + "&cmd=_notify-validate"
        req.ContentLength = strRequest.Length

        'for proxy
        'Dim proxy As New WebProxy(New System.Uri("http://url:port#"))
        'req.Proxy = proxy

        'Send the request to PayPal and get the response
        Dim streamOut As StreamWriter = New StreamWriter(req.GetRequestStream(), Encoding.ASCII)
        streamOut.Write(strRequest)
        streamOut.Close()

        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12

        Dim streamIn As StreamReader = New StreamReader(req.GetResponse().GetResponseStream())
        Dim strResponse As String = streamIn.ReadToEnd()
        streamIn.Close()

        If strResponse = "VERIFIED" Then
            'check the payment_status is Completed
            'check that txn_id has not been previously processed
            'check that receiver_email is your Primary PayPal email
            'check that payment_amount/payment_currency are correct
            'process payment
            Response.Write("<script language=javascript>alert('success!');</script>")
        ElseIf strResponse = "INVALID" Then
            'log for manual investigation
            Response.Write("<script language=javascript>alert('error1!');</script>")
        Else
            'Response wasn't VERIFIED or INVALID, log for manual investigation
            Response.Write("<script language=javascript>alert('error2!');</script>")
        End If
    End Sub
End Class