The ticket should come with a total weight of the passenger and their luggage. If I’m skinny let me bring 70kg bags.

Most of the time these models apply to the real world. Yes, in the cases where people act irrationally they don’t work. But a lot of economic phenomena can be explained through these simplified models.

and the shoppers are irrational

This is one of the first things addressed in econ classes: people are rational. In that case, the itdependsman’s comment is true. In the case you mentioned (and there are many similar cases), people make irrational choices, making the free market theories less realistic. Economists have to make these kinds of assumptions to make modeling the economy easier, but at the end people’s behaviour is chaotic and may be impossible to model at all.

Edit: behavioural economics is trying to understand the decision making process of individuals, and when they are more (or less) rational.