While he works at the A&P (the largest chain of grocery stores from 1915 through 1975), Sammy perceives the customers as "houseslaves in pin curls" with varicose veins that "map" their legs. He also sees them as "sheep" distractedly pushing their shopping carts down the aisles.
These women are dehumanized because of their repetitive and mindless routines. In fact, Sammy imagines them as animals. Two customers, for example, are likened by Sammy to frightened pigs in a slaughter chute as they vie for first place in the checkout line. So, when the young girls enter the store in their swimsuits, Sammy perceives them as a refreshing change from the usual customers. To Sammy, these girls are independent and vibrant.
When the manager of the store and Sammy's boss, Mr. Lengel, accosts the three girls in their swimsuits, telling them that they must leave the store because they are in improper attire, Sammy envisions Mr. Lengel as a representative of "The Establishment." Further, after listening to Lengel lecture the girls about proper attire, Sammy decides to quit in a rejection of the "stuffy" values of the management and as a gallant act in order to impress the girls. Unfortunately for Sammy, his gesture goes unnoticed by these girls, and he stands outside realizing "how hard the world was going to be [for him] hereafter."
I would add that Sammy also longs for that beautiful white body of the "Queen" who walks into the store. She represents a class to which he doesn't and won't ever belong (even after he quits his job). She buys the Fancy Herring Snacks while his family drinks "Schlitz in tall glasses with "They'll do it every time" cartoons stencilled on." But she also can break the rules with a certain audacity, and just as Sammy admires her confrontation with the "sheep" in the store, so does Updike, I think. He doesn't criticize them so much as, with Sammy, mourn and criticize the fact that such a gap exists, one which Sammy will not cross ove. Perhaps they are a representation of The American Dream (often embodied in American literature through wealthy, beautiful women) that the ordinary guy just cannot obtain.
Well, one is the disparity between the vacationing, monied class to which Queenie and her entourage belong and to the resident, working class to which Sammy is a member. Sammy longs to not suffer the same stagnant fate of his co-workers and to be accepted by the outsiders. The supermarkert, therefore, might be seen as a metaphor for those who must stock societies "shelves" and those who enjoy its spoils.