Updike delays a dramatic conflict halfway through the story. What does the author achieve by presenting Lengel's confrontation with the girls so late?

The story is really about Sammy, not the girls, not Lengel, not the shoppers.  So all the action in the story has to do with Sammy and his evolution as a character through this story.

"Ordinary events become pivotal as people examine their motives and reasons for their decisions and behavior. At nineteen, Sammy is ripe for experiences that will start to define who he is going to be. He discovers, as "his stomach kind of fell," that he prefers not to be a sheep who blindly follows the dictates of society."

The story is about Sammy learning a lesson about life, or the ephiphany, which he has at the end when he realizes what the consequences of his decision are going to be. Because he decided to walk out on his job to make a stand, to defy society, to be rebellious.

Sammy realizes that there are consequences to his actions.

"The girls inevitably stop their protestations, as Lengel expected they would, but Sammy quits—an act that Lengel could not have imagined ahead of time. To Lengel's credit, in spite of his stuffiness and self-importance, he shows Sammy patience. He does not yell or order him immediately out of the store, but warns him of the very real consequences of his act."

Like most young people, Sammy will not believe the words of an adult, he must learn them on his own.  He must come to feel the pain of his decision through his own experience, because that is the only way that he will learn what life is really all about.