Paypal order responder automatically order autoresponder

After someone orders something from my site via paypal, they should automatically be redirected to the download page. Sometimes this automatic redirection does not work. Because of this I always manually send them an email with the download details as soon as I receive their order, but if they order at night for example, often there is a delay of several hours before they receive this. Because of the delay, some customers complain to paypal. So I would like an automatic email with download details sent to them immediately after they order from me, and I need this to work every time! There are multiple websites that use the same paypal. The script should match the item number, email and then email the email on the order with the correct email with the download links.