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From Thikipedia
DPD makes it quick and easy to sell downloads. Sell downloads, automatically deliver your products, and run your own affiliate program with DPD. Get advanced selling features and options with no programming required.
Main features:
DPD allows you to customize your cart, delivery pages, and even emails with both simple WYSIWYG and advanced template editors. DPD allows you to sell your downloads on an unlimited number of websites, each with their own customizations and options. DPD protects your products from theft with a host of advanced security features so only the person who paid for your products gets access. In addition to download only products, DPD supports downloads with generated keycodes, keycode only delivery, and simple tangible goods.