Sales tax is the most obvious example of adding to the cost I've been shown, but it's everything. Here if there is a price on something that is the price you pay. Period.
If I have €5 and the price on the shelf is €4.90 we are all good, and I don't even need to know what country I'm in!

But is is more than that, if I take my car in to be fixed, they have to agree every cost they want to charge me in advance at no point can anything cost me more than I expected and agreed to up front.
Airline tickets, theatre tickets, hospital bills, TV ads, you name it, the price they state or advertise is what I pay, no ifs-no buts.