Another major theme of John Updike's short story, "A&P", is that of individual choice and the eventual consequences of the action. Most of the characters in the story have choices to make. Sammy, the cashier who narrates the story, makes the decision to quit his job at the A&P after his boss is rude to a customer. Mr. Lengel, the manager, decides to warn the young female customer about her skimpy attire. The teen, "Queenie," has to decide how to respond after Lengel's warning. Their decisions all have consequences, especially Sammy's, who must now face the rest of the summer (and perhaps longer) without employment and with an uncertain future.
The previous commentators seem to view Sammy in a negative light and come down on the side of society. I see the story a little differently.
To me, Sammy is the only character in the story that shows any individuality, and this is not a bad thing. The idea isn't that you better do as society tells you or there will be consequences--like a warning. The idea is that social norms are arbitrary and anyone with any brains or individuality has trouble going along with them. The societal norms are shallow and inflexible, and only sheep follow them.
The individuality Sammy shows is positive, not negative, and the choice he makes a good one, not a bad.
The future Sammy faces at the end of the story is one of constantly bumping up against asinine social norms.
In my opinion, the main theme of this short story is individualism. It is a story about what happens when people try to do "their own thing" rather than doing what society expects them to do.
In this story, the three girls are, to some extent, doing their own thing. Queenie has gotten them to go to the store in their bathing suits, which was completely unacceptable at the time. They pay for this by being humiliated by the manager.
Sammy also tries to be an individual. In his case, he tries to stand up for the girls against his boss. He, too, pays for his choice by losing his job and having nothing to show for it.
So I think the theme is that in our society, you have to do what is expected or pay the price.