Clearly one of the key aspects of this excellent short story is the way in which the point of view adopted by the author creates a specific tone and flavour to the language. The story is told from the point of view of Sammy, and thus he communicates to us what happens from his perspective using the kind of language that he is used to using. This of course results in the story being told in grammatically incorrect but colloquially correct English. You can also find lots of examples of slang and idioms in his account. Consider the following humorous example:
I rang it up again and the customer starts giving me hell. She's one of these cash-register-watchers, a witch about fifty with rouge on her cheekbones and no eyebrows, and I know it made her day to trip me up. she'd been watching cash registers for fifty years and probably never seen a mistake before.
Note how Sammy describes how the "witch" of the customer gave him "hell" and was a "cash-register-watcher." Later on, Sammy refers to housewives doing their shopping as "houseslaves" which indicates his contempt and distaste for normal, conventional life, which perhaps points towards the fascination that he finds in the girls and the way that they defy social norms through their act of coming into the supermarket in their bathing suits.
Thus the use of colloquial English that is not necessarily grammaticaly correct helps to create an image and impression of the narrator, Sammy, and also allows us to see his job and customers through his eyes, which is often quite amusing.