Also, while it is the norm in Japan to include the tax, there are some exceptions.
(Japan has 2 rates, 8% for certain items like food, and 10% for everything else).
Have you ever heard the term "Fronting"? In most stores, employees are required to individually ensure each and every item on the shelf is organized and pulled to the front for optimum display. They do this sometimes multiple times a day. In busier stores, you will have to restock high volume items multiple times per day.
The technology for digital pricetags has been cheaply available since the first kindle in the late 2000s. Most companies have avoided spending the money on buying them because the cost of labor in America is cheap enough that you can just have the normal employees do it every day. Digital pricetags are only now becoming common. Mostly because store companies are trying to figure out a way to charge you a personalized price that takes as much of your money as possible.
Yet at any given moment, every proprietor is miraculously able calculate the taxes due at the point of sale. The variety of tax regimes, and the fact that the amounts change doesn't impact the ability to calculate the final amount due.
Where in the US do the sales tax rates change so frequently? I've never seen this.
There is a caveat -- if you count the "sales tax holidays" in various US states, it means in those places, tax rates for some products change at least twice a year -- from 7% to 0% then back to 7%, for example.