To this point, because it is customary in France to greet the shopkeeper upon entering a shop (and vice-versa), the mistake many Americans make is to ignore the shopkeeper upon entering - since in many larger US cities the custom is "hope you don't mind if I just look, I'll ask you if I have any questions" which exceeds many Americans' French skills.

However, just ignoring the shopkeeper sets a rude tone which may well get returned, and some people come back saying "gee the French were so rude to us." Well . . . not exactly . . . but you can see how these things happen. So a few words can, in fact, go a long way.