In John Updike's "A & P," to find out where the exposition begins, it is important to understand what exposition is. In its entirety, we find that "exposition" can have one of two jobs as described below. The most important elements in the first definition below (with regard to how it is used in this story) is found in the second section regarding "background information about the characters and situation..."
...a form of discourse that explains, defines, and interprets. The word is also applied to the beginning portion of a plot in which background information about the characters and situation is set forth.
Another source notes that...
The purpose of exposition is to provide some background and inform the readers about the plot, character, setting, and theme of the essay/story...
What both of these definitions has in common is the sense that the in the exposition, the author lays the groundwork for the development of the plot. When we begin to read a story, the exposition gives us some context so that we can understand who the characters are, where they have been, and what is important about their presence as the plot moves forward.
Of the two examples you provide, I do not believe the quote about the "cash register watcher" is the exposition. The story is about Sammy and the three girls who walk in wearing bathing suits. The woman who complains when Sammy rings up her HiHo crackers twice is simply there to provide the reader with information as to the narrator's state of mind. While his state of mind is central to the story, the customer at his register is not. We do not hear of her again. Updike simply uses her, a "flat" and "static" character who provides essential information about Sammy, our main character and narrator.
However, when Updike's Sammy begins to describe the girls, we are presented with characters and setting immediately. We know there are three girls in bathing suits, that Sammy is working the cash register at the grocery store, and that within the first four or five lines, he has already described what he can see of their butts, and where their tan lines there begin and end. Updike speaks clearly enough through Sammy at the outset of the story, that we can infer that this is all about Sammy and the girls who walk into the store on a hot summer day—and what happens: this is the plot line. We will not know the theme until the end; we have no way of knowing if Sammy will befriend the girls, ask one out or embarrass himself completely in front of them and the entire store. We only know that for Sammy, this is the highlight of his day.
The beginning of the story, then, is where we find the exposition in this story.