Paypal integration (asp.net - visual basic) - repost

I would like to integrate into my website paypal payments when a visitor wants to download a zip file. The system is very simple: 1) the user clicks the download button 2) redirect to paypal for payment 3) when the user has paid (confirmed by paypal) 4) The file download starts automatically from my site The system must take into account any communication problems between paypal and the site and manage the safety of the download, that only the user who has paid can download the file from a private folder. The code must be written in dot. net (Visual Basic - [login to view URL]). I pay 30% only after delivery of the code and 70% after testing positive whole process of payment and download. Thanks and best regards.