>Yet, his site "Flattr" charges a higher service fee than any other service of its kind

Could you show me another service 'of its kind' as awesome as Flattr? Probably not.

Speaking of the the services not of its kind, Flattr's 10% is well reasoned. Appstore/Google Play charges 30%. Have you heard about Envato? They charge 50% on all sales until you make $70,000. Thousands of people make a living out of Envato and Appstore/Play.

Also don't forget that Flattr is dealing with micro transactions. With Flattr you can give $1 to 10 different content creators. Could you do that with Paypal? I think not. So, I would say that Flattr's 10% fee is well reasoned and acceptable.