Individual stores generally only have to deal with one. Set the prices at the store, and make them tax-inclusive while you're at it. This isn't rocket science.

Companies serve billions of web pages per second. We can't handle 12,000 tax calculations?

That’s illegal in a lot of places.

I doubt that most people in the US know the local sales tax. Let alone any change that may occur due to laws changing or traveling. I'd like to see the out the door price listed but that throws the 99 cent game off retailers like. Also I don't shop very often but Aldi US is the only place I've seen the eink price displays, the rest still have paper.

If its a wide region ad (can even be just across a metro area) showcasing a price then yes, they wouldn't know the price at a given store because the tax rates can change in less than a kilometer.

If there's a TV ad for a medium pizza for $10 at a chain they can't possibly know the tax rates for whatever actual store I'm going to go order from.

And the listing on a website won't know until I actually put in my shipping information.

Hi from Argentina! Here we unofficially deprecated the AR$10, AR$20 and AR$50 bill, so the smallest one is the AR$100 bill (~US$0.07). Every price include taxes.

What are they selling? Candies one by one? Inside the candy store everything is rounded to a multiple of AR$100. A single candy is AR$100. You many get an offer of 3 candies for AR$200, or 2 small candies for AR$100, or other fancy candies in packages of 13 for AR$1000. Everything else is more expensive, like AR$700 or more, but all multiples of AR$100.

The photocopy shop near my home has a copy for AR$120. They usually sell many copies, so a 20% is relevant. They have a stash of AR$20, but it's probably the only shop nearby. I also collect the AR$20 just to pay the photocopies, just to be nice to avoid finishing their stash and also because I don't know what to do with the AR$20.

I guess a single apple is probably a problem. It cost like AR$400-AR$500 depending on the weight. Someone very smart can learn to choose and apple with the exact weight to get a AR$499 apple and pay AR$400 :) Luckily inflation changes the price so it's difficult to learn. Also AR$499 will be illegaly rounded to AR$500. And most people will buy more than 1 apple, let's say that the total is AR$10,000 and AR$100 is only a 1% that is lower than the spoilage of rotten fruit.