> Some sales taxes are conditional at the point-of-sale

What does this mean, and why is it relevant to anything? I feel like you’re trying very hard to insist that taxes are impossible to calculate.

> there is no single tax-included price

There’s no single pre-tax price either. All of this taken together makes it trivial to tune the post-tax price to round to the nearest nickel even if your tax authority hypothetically insists rounding is forbidden. The POS system says you owe 31.67 based on the ticket price and the applicable tax, rounds to a target of 31.65, and then applies a 2 cent discount pretax to your purchase to make this work out.

Hell, gas stations have proven prices can be in fractional cents so you could even calculate and apply a fractional discount if you find an edge case where you can’t discount whole cents and still round to the nearest nickel.

> The rules of multiple authorities apply to single jurisdictions.

There is no federal sales tax rule, so there is at most one soveriegn authority whose rules apply to any jurisdiction as to computation and collection of sales tax, any other authority is a dependent one, whose policies can be (and probably are already) constrained by the sovereign's own rules governing sales tax, which can be the level at which solutions are imposed, if necessary. "Thousands of independent authorities" is not a reality.