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Impossible to do on the Steem blockchain unless you authorize a bot to transfer money from your account through SteemConnect. I'd prefer not to do that. But maybe a smart contract on SteemEngine could do that in a decentralized manner. But if you get a bill, at least you can check it first for errors and not pay until possible mistakes have been sorted out.

I so much agree with this comment. One of the things most people enjoy on the platform is that no-one except them has control over their account so, an automatic taxation would defy that.

As much as I would have loved your idea, it's so painful my country's eyes are not yet open to the blockchain world.