The plot, setting, and characterization certainly contribute to the main theme in A&P: the allure of female sexuality and the emotions it elicits in the male psyche.
The author liberally uses direct characterization to draw our attention to the girls' physical beauty. Here are two examples:
She had on a kind of dirty-pink—beige maybe, I don't know—bathing suit with a little nubble all over it and, what got me, the straps were down. They were off her shoulders looped loose around the cool tops of her arms, and I guess as a result the suit had slipped a little on her, so all around the top of the cloth there was this shining rim.
With the straps pushed off, there was nothing between the top of the suit and the top of her head except just her, this clean bare plane of the top of her chest down from the shoulder bones like a dented sheet of metal tilted in the light. I mean, it was more than pretty.
The quotes above describe Queenie, the narrator's favorite of the three girls who just walked into the store. We are left in no doubt as to the narrator's feelings about the girls; his thoughts are made plain to us through the vehicle of indirect characterization.
Queenie puts down the jar and I take it into my fingers icy cold. Kingfish Fancy Herring Snacks in Pure Sour Cream: 49¢. Now her hands are empty, not a ring or a bracelet, bare as God made them, and I wonder where the money's coming from. Still with that prim look she lifts a folded dollar bill out of the hollow at the center of her nubbled pink top. The jar went heavy in my hand. Really, I thought that was so cute.
"Oh Daddy," Stokesie said beside me. "I feel so faint."
"Darling," I said. "Hold me tight."
The quotes above highlight the narrator's sexual attraction to Queenie; her allure is clear in her sensuous mannerisms and behavior. The setting also reinforces Queenie's physical beauty. Here's an example:
You know, it's one thing to have a girl in a bathing suit down on the beach, where what with the glare nobody can look at each other much anyway, and another thing in the cool of the A & P, under the fluorescent lights, against all those stacked packages, with her feet paddling along naked over our checkerboard green-and-cream rubber-tile floor.
The plot also reinforces the theme of the story. We learn that Sammy eventually quits because he's upset that Lengel embarrassed the girls. For his part, Lengel tries to talk Sammy out of resigning, but the latter will none of it. As he leaves the store, Sammy looks out for the girls, but they are nowhere in sight. Despondent, Sammy suddenly realizes that his decision may result in negative consequences for his future happiness. So, the plot reinforces the theme perfectly. The girls' raw sexuality inspires strong feelings in Sammy and leads him to question the mores of his society.