It seems to me that French culture finds a pretty happy medium between the two extremes: you aren't expected to engage in a "full" conversation asking how the other's day is, but a simple "bonjour monsieur/madame" and "merci monsieur/madame, au revoir" is more than sufficient.

To me this is the optimal point between being sincere and being socially cordial with a stranger, especially one whose shop you have just entered to do business.