That doesn’t actually matter. You are making excuses for why the tax system is too complicated to include tax, and I’m telling you they’ve already solved that issues because the EU, in which most counties share the same currency, companies are required to include tax in the displayed price and show the customer how much of the purchase is tax.
I work with Point of Sale companies and know this problem is solved. It is not too difficult to achieve from a technical or practical perspective, so you’ll have to dig up some other reason this is somehow unachievable.