The story was published in 1961, almost sixty years ago. However, many of the details of the A&P show that small-town chain grocery stores have not changed that much over time. Some details that seem true to life are the following:
I'm in the third check-out slot, with my back to the door, so I don't see them until they're over by the bread.
I stood there with my hand on a box of HiHo crackers ...
all three of them went up the cat-and-dog-food-breakfast-cereal-macaroni-rice-raisins-seasonings-spreads-spaghetti-soft drinks-crackers-and-cookies aisle
our checkerboard green-and-cream rubber-tile floor.
they come around out of the far aisle, around the light bulbs, records at discount of the Caribbean Six or Tony Martin Sings or some such gunk you wonder they waste the wax on, six packs of candy bars, and plastic toys done up in cellophane that fall apart when a kid looks at them anyway.
As I say, we're right in the middle of town, and if you stand at our front doors you can see two banks and the Congregational church and the newspaper store and three real estate offices ...
Paying close attention to the details helps establish this as a small, working class grocery store, not a giant flagship in the A&P chain. For instance, the three girls go up an aisle that holds many different kinds of items that would be separated into different aisles in a larger store, such as pet food, seasonings, pasta, and soft drinks.
Specific details add to our sense of realism, such as the HiHo crackers, the green and white tiled floor, and the records by the Caribbean Six. Simply mentioning "crackers," "tiled floors," or "records" would not place the reader as vividly in the scene as the specifics do. The details reinforce the idea that overall the store is a little bit cheesy and discount-like, making Queenie's choice of nothing but a jar of herrings, an upper-class purchase, stand out all the more. By providing all these details, Updike shows us the class difference between Sammy and Queenie—he doesn't then have to explain it to us.
The fact, too, that the details of the store are all visual adds to the idea that the A&P is simply a slightly tacky backdrop to a bigger drama that rivets Sammy. The ordinary, down-market details also contrast with the glamorous details of the life Sammy imagines for Queenie's family. These hum-drum details help us to understand why Sammy might want to escape this life and aspire for more—even though, as he acknowledges, he chooses a harder road when he quits.