Welcome to the land of mental arithmetic!
During your stay in the U.S., to know how much something costs you will need to mentally add the sales tax, which is not generally included in the price displayed.
During a California road trip, for example, sales tax is around 8%. If you see an article at $100, it will cost around $108.
Things are even more complicated in restaurants, where you also need to add a tip (about 15%) which is not included in the menu price. For example, if your meal costs $20, don't forget that around $1.60 in sales tax will be added to your bill, and you will need to leave a tip of about 15% (before tax) for the wait staff, another $3!!!
Here are the different sales tax rates for several U.S. states in 2020:
New York: 4.00%
To learn more, read the article Calculate the sales taxes in the USA which clearly explains the calculation by state and even offers online calculation tools.
In the U.S., tips are never included in the price. Even though a tip should depend on the quality of the goods or services received, it is customary in the United States to leave a tip equivalent to 15% of the total bill, before taxes, in restaurants and bars. You can leave the tip on the table or give it directly to your waiter/waitress.
It is also common to leave a tip for the housekeeping staff if you are satisfied with their service.