I'm sure there's a script or product that can do that somewhere, although I don't know of any off of the top of my head. However it would be really easy in PHP - when the user completes the order you can just grab his IP address and add it to an SQL database table, then have the download page disallow connections from any IP not in that database. This locks out everyone besides your buyer, while still allowing him to revisit the download page if he needs to for whatever reason.