oh, that's hefty indeed. sorry to hear that. :/
I got this from Steam panel:
"Payments made to you may be subject to US tax withholdings per US tax reporting regulations. Developers may be eligible for a reduced or exempt rate of US tax withholding on units sold to US customers if your country of permanent residence has an income tax treaty with the US. At the end of the interview, you will be notified of the US withholding tax rate, if any, that may apply to you."
So, this withholding tax should apply only for sales for U.S.A. customers, right?
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.