'Flair' = Metaphor for Tipping Culture
I just realized - 'Flair' works EXACTLY like 'tipping culture' in some effed-up countries!
Think about it - you are EXPECTED to tip, but not technically REQUIRED to do it. So if you don't do it, you end up in 'trouble', but they can't tell you that you HAVE to tip this or that amount - it's up to 'your discretion', so no one can specify an exact amount, but at the same time, you are expected to pay this 'undisclosed amount'.
The boss can't tell her how many pieces of flair she has to wear, because she is supposed to be able to figure out for herself, and the boss can't REQUIRE her to wear more than 15 pieces of flair, although he clearly EXPECTS it.
It's exactly like tipping, isn't it? You are not exactly, specifically and logically told what you are supposed to give, you are not even told that you absolutely have to do it, but if you don't do it, and if you don't do it with the right amount (or more), you will be 'in trouble'.
They will spit in your food, they will manipulate things, false-accuse you, get you fired or whatnot, if you don't tip.
The boss obviously can't resort to such underhanded tactics, but the 'stern talking to', trying to HINT that she should wear more flair, is pretty much the equivalent - both are 'trouble'.
People that live in countries that actually pay decent wages, understand that someone shouldn't get angry at the customer - after all, they are not lawfully required to tip - but they should get angry at the BOSS, who clearly isn't paying them enough, because they expect customers to handle part of their pay as tips (which is just absolutely insane, and makes no sense, but because it's the 'norm', people don't notice the madness of it).
In my opinion, this sort of things are really weird and should be abolished. The 'flair'-requirement is probably the most misunderstood part of this movie, but also, a really good metaphor for how insane the 'tipping culture' is. You are expected, but not required, and no one tells you what the correct amount is. This is exactly like the whole flair-thing.
This also leads to uneven results, doesn't it? Everyone 'figures it out' in their OWN way and with their own logic and math, some people take into consideration how long the trip was, how hot the food was, how long a time it took, whether someone smiled or not, what kind of tone they used and a heckuva lot more variables.
Others just use the same amount every time, some people actually calculate percentages, trying to be as reasonable and fair as possible, without being taken advantage of and so on.
It's a crazy thing that makes no sense - why don't the bosses just PAY FAIRLY, so we wouldn't need it? It's not, and it shouldn't be customer's responsibility to pay someone's wages.