Since at one point in my life I was a teenage boy, the entire story stands out as particularly true to life.
"A & P" narrates the internal monologue of a 19 year old store clerk when a group of three bathing suit clad girls walk into the store. It strikes me as particularly true to life that once Sammy sees that the lead girl, Queenie, is extremely attractive, that is all that he can think about.
It also strikes me as true to life that most of Sammy's descriptions of the girls focus on their individual body parts. I teach a media class, and one of the units that I teach is sexual objectification of women. It's all over the place in modern media. One key trait to a sexually objectifying image is that the image will focus on certain parts of the female anatomy and generally leave the face out of the picture. That's how Sammy treats Queenie. He describes her legs, her feet, her shoulders, her tan, and her breasts. There is a single description of her face.
"a kind of prim face."
Updike's story is the literature version of most TV advertisements of today.
The last part of the story that strikes me as true to life is Sammy's attempt to be a hero figure to the girls. He quits his job as a way to stand up to his boss's treatment of the girls. He thinks that it will win him some points with the ladies. Unfortunately, it fails, because they don't even notice. I feel that is true with a lot of males. They imagine themselves as a sort of heroic savior to a damsel in distress, but it rarely works out that way in reality.