Aside from silly "sales-tax-holidays" I've never heard of the sales tax rates changing so frequently. I'm curious to know where this is happening "multiple times per year". Here in MA it's been the same rate for 15 years.
And for the sales-tax-holiday situation can't a shop just say something like "everything will have x% taken off at the register" just like they would during a typical "10% off all items" type of sale?
What do you think all the people collecting a paycheck in a store do? Stand around and look pretty?
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.
Around me most stores change their price tags weekly, if not more often. They either have a sheet with new tags or a small printer and scanner and they swap out the tags on the shelf.