I highly recommend Linklok Paypal from Vibralogix for automatic delivery of digital products. This program works seamlessly with Paypal and makes the business end of selling a much easier process for sellers of downloadable goods. Vibralogix has went above and beyond to make the set up process simple and incredibly user friendly. And they have superb tech support, too! I have used this program for years and I really can't say enough to give it the cudos it deserves.
If you sell or plan to sell digital goods, you won't be disappointed. Click Here to visit their website.