Sammy belongs to what's sometimes known as the "blue-collar" class. The name comes from the blue collars on the uniforms worn by many factory and industrial workers. These workers, at least in 1961, generally earned less than so-called "white collar" workers, such as clerks, accountants, and other office workers (on the low end) and bankers and lawyers (at the high end).
Updike makes these class distinctions explicit in an interview, and there are hints of them in the story. Sammy's class is revealed through his job, his fears about the future, and his family's use of beer glasses to mark big family events. His impression of Queenie's economic class is based on his finely honed people-watching skills. He makes an educated guess about her class by taking into account her dress, her bearing, what she buys, the way she talks, and the way she responds to Lengel's attack on her clothing.
Updike claims that this difference in social classes is the main reason Sammy quits his job. He says Sammy's "gesture of quitting has to do with the fact that she was rich and she was poor, as he sees it" (source).
Sammy seems to be looking for a girl who will show him things he's never seen and take him places he's never been. Updike suggests that not just any girl would have inspired Sammy to such action. What do you think? If a pretty girl from Sammy's neighborhood came into the store and was treated badly by the manager, would Sammy have quit?