Could be because I’m in Europe but I regularly buy from GOG (an European company) and from Steam (an American company) and this does not happen, what you see is what you pay. I’ve just bought Dwarf Fortress on Steam 20% cheaper than here (same displayed price only at checkout Itch.io increased the price by 20%). And Steam price already included 20% tax!!
I occasionally buy directly from the developer site (Factorio, RimWorld, KeeperRL, CogMind), HumbleBundle and Fanatical and this does not happen either, it’s definitely something weird with Itch.io, they other double charge for tax or their prices are 20% higher.
My feedback is that despite the attractiveness of the site, offering higher prices than everybody else is a significant hurdle. One or two games, that you can’t find anywhere else, is fine but doing consistent / holidays shopping here is unfortunately out of the question.