As an American consumer, I don't find this confusing; taxes change even among individual small cities, and they can change multiple times per year. The taxes really have nothing to do with my actual purchase: it feels more like a payment processing fee, a "cost of living". Yes: I realize that as a non-American consumer this sounds insane. People have different expectations and subtle details can change opinions; I pretty much never hear an American complaining, however, that they didn't realize there was a tax on something.