"A&P" author John Updike is known for the idea that seemingly ordinary aspects of American life (like grocery shopping) are actually quite fascinating. He called this, famously, "giving the mundane its beautiful due" (source). Updike wanted readers to see the beauty and magic of life, so he tried to describe everyday things using the most clear but beautiful language possible. You can see this idea at work in his poetry and literary criticism as well.
In "A&P," part of the beauty comes from narrator Sammy's distinctive voice. For one thing, he skips back and forth between past and present tense. The novel's first sentence is, "In walks these three girls in nothing but bathing suits" (1). (Note the grammatical error, folks. If this were the SAT, you'd want to change "walks" to "walk," but the "mistake" helps establish a casual, realistic style.) This opening is present tense, as if Sammy is commenting on events as they happen. From that point on, though, he switches back and forth between present and past tense, again giving us a sense that we're reading his unedited thoughts and that he is as unsure of what's to come as the reader is.
But in the middle of the story, Sammy tells us, "Now here comes the sad part of the story, at least my family says it's sad, but I don't think it's so sad myself" (11). Aha – so Sammy does know how the story ends, and he's telling it, thinking it, or writing it sometime after.
Nowhere in "A&P" are we told whether Sammy has written his story down. The deviations from traditional rules of grammar and the shifting back and forth between past and present tense make the story feel like it could be (a) a stream-of-conscious narrative, where we are meant to be inside a character's mind, reading his or her thoughts, and/or (b) a story that Sammy has been telling out loud.
But take this great sentence:
I uncrease the bill, tenderly as you may imagine, it just having come from between the two smoothest scoops of vanilla I had ever known were there, and pass a half and a penny into her narrow pink palm, and nestle the herrings in a bag and twist its neck and hand it over, all the time thinking. (20)
There is a polish in this sentence that makes this sound written down, like it's been edited and worked over into an understated perfection. Ultimately "A&P" uses a variety of stylistic techniques to highlight Sammy's desire to break free from what seem like inescapable traditions in his community.