... depends on the business model of selling/buying ebooks: if you sell (like Amazon) a book for 10US$ then everything is included for the end-user. You could also sell a book for 10 US$ and have the end-user pay the download extra. This may be a show stopper if the end-user has no flat rate and you may loose control of the sales chain.
A good example is Apple who want to control every bit of the chain and actually are on their way of doing it. Very clever.
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