It all depends on region. Certain US states we do charge tax in, because we have office locations in those states. I don't have the full list, but off the top of my head I recall we have offices in CA, WA, TX, PA and maybe a few others. In addition, international tax (e.g. VAT) is calculated differently and is included in the price in some places (namely in the EU) while US tax is added on top. In any case, the tax/VAT percentage can be different from state to state or country to country. I don't think there is any error, but feel free to send me a PM with your specific question and I can try to look in up. Thanks.
