Wow, isn't it in USA? Do I understand you correctly: I see a "price" on a product, and have to manually multiply it by 1.2 (or whatever taxes are in USA) to know, what it actually will cost me?
For example, in Massachusetts, the sales tax on most products is 6.25% so when purchasing most things you must multiply by 1.0625 to figure out how much it will cost. Most uncooked food does not have sales tax, but food served by restaurants does.
When eating at a restaurant, you're supposed to tip 15-20% on the total bill pre-tax, though a lot of people tip on the post-tax price depending on how cheap/generous they are. Many restaurants calculate the tip for you, but certainly not most.
In the US if you go out for dinner and order the caesar salad (listed at $6), the skirt steak (listed at $15), and a glass of the house red (listed at $8) you will be presented a bill for $29 + sales tax, in NYC for example the bill would come to $31.58. Then you will be expected to leave between 15% and 20% on top of that as a tip, so generally you would be expected to pay $36-$38 for that meal.