I suspect the answer is instead to establish two tax rate calculations: One for in-person shopping (like VAT) -- you pay the tax according to local rates, but

I suspect the answer is instead to establish two tax rate calculations: One for in-person shopping (like VAT) -- you pay the tax according to local rates, but it's factored in already. one for online shopping, ("E-VAT") -- you pay a national rate tax and the seller is responsible for paying gross sales based on that percentage to the state and the rest goes to the IRS. Problem comes with the Sin Taxes that have been established. For instance, in Seattle, sugared drinks MAY incur a tax depending on what kind of store you bought it from (e.g. the normal costco has to tax it but the business costco doesn't), but that doesn't affect some folks and then there's tax-exempt organizations like churches that can have their sales tax waived and then there's states where sales taxes are a majority of the income is from sales tax but only if you're local and oh god it gets bad.