I’m French and I have never heard anyone say “salut” to greet a clerk. It’s just too familiar. You either say “bonjour” or “bonsoir”. I’d recommend you to just repeat the word they use to greet you, since generally they’re the first to talk in order for you to know that it’s your turn.

It’s true that “hi” is less formal than “good morning” and “salut” is less formal than “bonjour”, but they are not used in the same circumstances. You pretty much never say “salut” to someone you don’t know.

You can say “salut” to a friend of a friend who you’ve just been introduced to. You might say “salut” to a fellow member of an organization (it depends on the organization) even if you don’t know them personally. But don’t say that to a clerk.

There aren’t many contexts where “bonjour” is too formal. The only difficulty with using “bonjour” is the time of transition from “bonjour” to “bonsoir”.

Most have been say already, but I’ll add that it’s not uncommon to great a person by adding “Monsieur” (sir), Madame (madam) or Mademoiselle (miss): bonjour Madame, etc; for example to great the baker from whom you buy your every-day bread. It’s more polite but not necessarily as formal as one would think of when saying for example “Good day Sir” in Great-Britain.

Again, simply put, it’s very common in the USA and some other English-speaking countries to say “hi” to people even those you’ve never met, it’s not in France; unless, as has been pointed the person is much younger than you and you want to be “cool”. But even to a cool looking young clerk, you wouldn’t say “salut” (unless you personally know the person).

While all the answers recommending to use Bonjour are absolutely right, if you’re ever in Valais, in Switzerland, there Salut is fine as well and clerks will probably greet you with it anyway.

I am French. Very easy :
Important persons (your boss, president, high ranking official, ….) : Bonjour/Bonsoir.
People you know well or very well : you can use Salut.

But by default, I’d say Bonjour. It is the formal way, but this is not to formal… I say bonjour to friends or even my parents sometimes.

You will never make people think “wow hey relax we don’t know each other” with Bonjour…

Maybe you will with Salut…

For example, my hometown is 5 minutes from Belgium, clerks will say Salut to customers… not in France…

In restaurants/cafes/supermarkets, etc:

You can personalize by adding the first name “Bonjour Paul” to be both polite/respectful and friendly without being too formal. This is to be used only if you know the person, I mean if the person is both able to recognize you and you had already talked to him/her (this is more usual in cafes/restaurants than in supermarkets). In case of doubt, use the simple “Bonjour” and enlighten it with a light smile.

In luxury brand shops:

Simple “Bonjour” is the best. Personalization by first name is possible only if your social status is high and you are a frequent customer.

Bonjour is used everywhere with anyone, you use it at work , at home or with your friends. salut is similar to Hi! would you say Hi to your boss or when you to the post office? may be but in France (I am french) I cannot see myself saying Salut when asking to buy a book of stamps at the post office.
French people are more rigid than english people, even in the shops, the cashiers in France for example are nor friendly like in the Uk. Working people in the Uk will interact will the customers and will chit chat, in France they just do not do i. Once I was in France, i said Hello to the cashier and I said how are you today, she was surprised.

I’ve also lived in many places (medium and small towns) ‘n france and haven’t heard a a clerk being greeted with familiar terms. I’m sure it happens if a relative or close friend happens into the shop or whatever but I’d just always use Bonjour, as everyone else seems to do. I love it when they sort of raise the bon part, sounds so cheery.

I read that a cafe-owner in the South put up the following price-list, to discourage rudeness.

• Un café … 8€
• Bonjour, un café … 4€
• Bonjour, un café s’il vous plaît … 2€

In all cases you mention (supermarkets, cafes or restaurants, luxury shops), when adressing people specifically, simply saying “Bonjour” is not very polite, although acceptable.

A well educated person says:

• “Bonjour Madame” to a woman
• “Bonjour Monsieur” to a man
• “Bonjour Mesdames” to several women
• “Bonjour Messieurs” to several men

• “Bonjour Messieurs Dames” to a group composed of at least one man and one woman.

• “Bonjour Mademoiselle” is old-fashioned (as replaced by “Bonjour Madame”) but you can still hear it sometimes in restaurants when someone is calling the waitress.

I’m sure that such formulas will sound too formal to some people, but it’s really a question of habit and does not prevent being natural and warm with people.

As Jean-Baptiste wrote, you can use “Bonjour/Bonsoir” followed by the first name once you know the person. Using such formular requires a personal and friendly relationship. Eg. with an regular customer. “Bonjour Paul”, “Bonjour Marie”, …

“Salut” would be too popular with people that are not friends, children or members from your family.

You can use “Bonjour” if entering into a popular restaurant/bar and adressing to everybody. You may also hear “Salut” in restaurants from ski stations, for instance in Valais (Switzerland), but often because native people know well each other.

Nico says that you can say ‘salut’ in the Valais to store clerks. In Geneva (where I used to live), that would never be done. It’s just too familiar. There, the greeting will be ‘bonjour, monseiur/madame.’ The complexity in Geneva is with the valediction, but that’s not part of the question here (though it was part of someone’s answer). In Geneva, very precise indications of time of day may be given. I even once heard ‘bon fin de l’après-midi, monsieur,’ a valediction that I had previously joked about as only theoretical.

Nico says that you can say ‘salut’ in the Valais to store clerks. In Geneva (where I used to live), that would never be done. It’s just too familiar. There, the greeting will be ‘bonjour, monseiur/madame.’ The complexity in Geneva is with the valediction, but that’s not part of the question here (though it was part of someone’s answer). In Geneva, very precise indications of time of day may be given. I even once heard ‘bon fin de l’après-midi, monsieur,’ a valediction that I had previously joked about as only theoretical.

I live in Quebec. The word “bonjour” is neutral, you can say it to everyone, even to a kid. Here, in Quebec, if you saw that clerk many times, it’s adequate to say “salut”. Otherwise, if the stranger is younger than you, it’s adequate to say “salut”, but it’s not adequate to say “salut” to a doctor, a boss or a principal. If you don’t want to distinguish “bonjour” and “salut”, you can always say “bonjour”, even to a kid.