Anchordesk writes about micropayments and try to explain why no one uses it. Clay Shirky's explanation is better. He catches the core problem, "users want predictable and simple pricing".

If you don't know what you'll get for the money you don't pay. The buyer must be able to predict the outcome of the transaction.
No rating system handles this well yet and not many sites tell you what you're gonna get, before you get it. Amazon, of course, is trying hard.
Subscription is a way to let people know what they're going to get. They've read one issue of a magazine and therefore have a pretty good picture of what they are paying for.

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