In the United States I always see products listed like "$100 plus tax". This might be a stupid question but does that mean with tax calculated in it comes out to $100, or are they trying to say that it is $100 and tax will be added on top of that? I would have thought it is the former, but I have been wrong and have been charged tax. It seems a bit ambiguous to me.
$8.99 plus tax means: $8.99 plus an additional amount for tax. You pay more than $8.99, how much more they will compute for you automatically when you check out. The merchant keeps $8.99 of what you pay, and the rest they send to the city, county, state, or other sales tax district involved.
Fortunately cash registers are computerized. But some of us old geezers may remember how it was when computers took up an entire room, and a measly retail store did not have one.
You may sometimes see $8.99 no tax (for example paying on the internet to a merchant in a different state). Then you pay only the $8.99. And it is then your responsibility to pay the appropriate tax to the tax districts where you live. Some states make this easy to do; others less so.
Your original question, before editing, said "with tax." This is probably equivalent to the phrases "including tax" and "price inclusive of tax." (This is the opposite of "plus tax.") Here is an example from the woocommerce online store software, which I found with a google images search:
As you can see, there is a settings prompt that reads, "Prices entered with tax." The corresponding radio button reads, "Yes, I will enter prices inclusive of tax."
Note, in my experience as a consumer in the US, the only situation that arises in everyday life in this day and age in which I occasionally make purchases without tax being charged, either explicitly or implicitly, is with online shopping. Sometimes the seller charges tax and sometimes not. Once a year I have to fill out my state tax declaration ("tax return"), and there is a question there that asks me to estimate how much money I have spent in online purchases. Then there is a calculation step to apply the state tax rate to that amount. This is where the state captures the lost sales taxes (lost to out of state online retailers).