Just stopping by to mention, for the non-Americans, that taxes as in the plural is important here: Sales taxes here in the US vary at the municipal, county, and state levels and any or all can apply towards your purchases at the same time.

From earlier HN threads, it seems that:

- When you contact government offices to ensure you're paying the right taxes on the right things, it's very difficult to get a real answer. Many of them just outright do not know, and rely on "a good-faith effort".

- The situation is so convoluted that their are specialist companies which sell you the idea that they can take care of it for you, yet want you sign a contract that says they're not liable for any problems.