You could use our Updates/Newsletters service, but that would only send updates to ALL previous buyers of a given product, with no way to limit it to only buyers in the past week:
http://www.e-junkie.com/ej/help.updates.htm
Aside from that, you can go to Seller Admin > View/Download Transaction Log to find last week's buyers, and then just issue each of them individually a new download link from your Seller Admin > "Send free download link".