I'm not sure why the article says this is so hard for retailers to figure out. They already round.

When they apply 7% sales tax to a $1.99 purchase, what do they charge you? $2.1293? Obviously not.

Just do whatever they already do.

The article mentions several practical and legal issues, and your comment does not address any of them.

Also, "whatever they do" currently is rounding to two decimal places - that doesn't help us here. I'm not saying the software changes required are challenging, but I'm sure there are lots of POS systems that are not properly maintained anymore that will cause issues for a lot of smaller merchants.

The problem is that the rounding you are asking them to do is illegal in a few states.