The chain grocery story is now a fixture of American life. In the 2000s we have 24-hour stores, self checkouts, and vast arrays of basic and specialty items, from rice and beans to expensive patio furniture. The A&P grocery store chain was one of the pioneers in this industry, paving the way for the superstores we see today (source).
In Sammy's eyes, the A&P he works in reflects the conformist tendencies of his community. He sees the store patrons as "sheep," or followers, rather than independent thinkers. In an interview about the story, Updike says that in those days in America [the 1950s and '60s], "people by and large conformed and were expected to conform" (source). Sammy and the girls in bathing suits represent a rebellion against this conformism. In the interview, Updike refers to "voices of dissent" in the 1950s and '60s, like Allen Ginsberg, Jack Kerouac, and James Dean – people willing to risk embarrassment, quit their jobs, or otherwise take a stand for individual freedom.