I found US sales tax baffling - I live somewhere that requires the display price to be the out-of-pocket amount.
I think there is also a policy view that taxes should not be hidden, that there is a public interest in making the public aware of precisely how much they are paying in taxes. Taxes that are baked into the price tend to be forgotten about by the public.
Strictly speaking, you don't have to change the total price if the tax rate changes, you can change the base price so that the total comes out the same. This is how prices are typically set in Europe, though vendors can of course change prices in response to tax rate changes, but it doesn't normally happen automatically.
Now, you might argue this hides the tax even more, if the customer doesn't even notice a price change when the rate changes, but one could also argue the latter is a good thing for the customer (if the business decides to eat the cost, at least for a while).
I have never seen anywhere that advertised the tax rates at a store when I've visited the US.
Taxes being different, not baked in, and not advertised just makes the whole system appear random.
It's a solved thing, for the cash round down, for the credit/plastic/online pay as is.
If you remove $0.01 then round down to the nearest .05, if you remove $0.10 then round down to nearest 0.10; so both $9.99 and $9.95 would be $9.90. Simple.
