One-time download upon purchase. Options

[zgenz]

I am going to start selling vector illustration packs on my website because of the overwhelming demand for them. I have decided to use paypal to process the transaction, but what i need to do is figure out how to get the actual .zip or .rar file containing the .AI(illustrator) file and the pdf containing terms of use to the customer upon transaction completion. Paypal has an option to forward the customer to a certain link upon successful completion, and i was wondering if it would be possible to generate a script that would allow someone forwarded to a link to be able to download the file once, and only once, to prevent that person from giving that download link to everyone.

Any help on this subject, or maybe an easier method to accomplish what im trying to do would be much appreciated