I don' t know about Steam, maybe they have a branch in Europe(/brazil?, I doubt it ;) ). If they do have a branch in your country, you won't have to pay the tax cut I guess.
But with Itch (and I assume steam as well honestly) any payment (either by US, European or Brazilian citizen) is made to an American Company (Itch, Steam, ...). So the money you make, is made in the US. This means you pay this tax on all money you made.