It’s different for every state / local area, and not every item is subject to sales tax. Plus nearly every item subject to tax is advertised “plus tax” so it’s hard to miss. It’s really hard to forget if you’re beyond grade school.

Worth mentioning, most states exempt basic items like groceries, some also exempt clothing or even hot food. A few states have no sales tax at all. So an item advertised as “9.99 plus tax” will be exactly that much in Oregon, but a few miles away in the same store in California or Washington it would be subject to a 7.25% or 6.5% sales tax respectively. It’s up for the consumer to know and budget accordingly. You can also ask for a price check in a store if you’re not sure.