Yeah if merchandise is sitting on a retail shelf long enough that the tax changes multiple times before it's sold, you need to rethink your stocking choices.

I don’t see why we can’t have both. Have one price that is the grand total, and in a smaller font, display how much of that grand total is the tax. That way people know how much the thing will cost upfront, and also know the tax amount.

In the USA, I have no idea. In the EU, politicians don't want that because that'd surely lead to a revolution, especially in the east.

Sales tax in the USA differs across states, and even across counties. So it could change fairly regularly.

VAT differs wildly across EU countries, including whole different schemas of the tax. And not only that, we also speak two dozens of languages. What you're saying is simply not a problem - come and see. I've never even thought that it could be seen as a problem because the solution (don't move tags, just the product) is so obvious.