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Is sales tax for online purchases based on billing- or shipping address?
From my understanding as a seller, and having read through Amazon's 8 page calculation methodology document, the default is the ship to address, however the seller still has the option to charge the tax or not, only charge the state rate and local (city, county, district, etc.) rate(s), or even set their own self-determined default tax rate.  In other words, the seller has a lot of control in determining what rate they use and the billing and shipping addresses may not even matter.  Just remember that whatever tax you pay to Amazon, your state will probably still hold you responsible for calculating and reporting any additional use tax, based on your location.  And if the seller does overcharge for tax you may have a right to request a refund from them.