The Paypal Assistant uses Paypal's IPN to give you immense flexibility over your post sale activities. This script allows you to automate just about anything after the sale. You can use this script to deliver digital goods upon a purchase, post data to another script, execute commands on your server, or even execute Perl code. You can setup conditionals to do different actions. For example, you can set actions for just users from the UK. You can execute code to automate a hosting service. The possibilities are endless.