I am looking for a script that I can use in conjunction with selling e-products. I want it to have the ability to generate a unique that will expire in x number of days and email/display on screen after a customer has made a purchase using paypal.

I have an eStore script, but I am more looking for something I can add a file and display a link i.e.) www.mysite.com/?buy=1 or something.

Any suggestions for such a thing? Can be free or paid