Are there any subplots in A&P?
I would offer that there is a subplot in A&P. The main story has to do with Sammy, the protagonist in the story struggles with his feelings of confinement in the A&P, the place where he works, which to him is also symbolic for the establishment. At his age, Sammy is dealing with his feelings of rebellion against the establishment when he makes fun of the people who shop in the store, the absolute meaninglessness of the job, the repetition, for Sammy, the absolute boredom that he experiences as he does his job.
This is going on before the girls walk into the store, so when the girls enter, they create a subplot or a storyline that is subordinate to the main plot, but also contributes to the main plot, it is a dramatic subplot which heightens the concentration level of the feelings of rebellion in Sammy. If Sammy was a stick of dynamite, figuratively, waiting, knowing that he would explode at some point, think of the girls entering the story as the match that lights the fuse. Their presence in the store sends Sammy into full blown rebellion, actionable rebellion, not just thoughts of rebellion. Without this inciting incident, Sammy might have mindlessly worked at the register in the A&P thinking about rebellion but never acting on it.
The subplot is dramatically linked to the main plot, which is Sammy's feelings of rebellion, the girls entering the store dressed only in bathing suits opens up a whole new line of rebellion which Sammy immediately accesses and becomes emboldened by, so much so that he quits his job.
So the two plots are closely linked dramatically, the subplot contributes to the climax of the story and therefore is the type of subplot that is closely associated with the main plot, not a parallel subplot.
Most short stories do not have subplots and I have to say that "A& P" does not appear to be an exception. All the action that is going on plays directly into the main plot. It is difficult, though not impossible, to have a subplot when the story is in first person as A &P is.
I would say, however, that the story has more than one theme. On one hand, it is a "coming of age" story for Sammy, but on another level, it shows a clash of values between aged and youth.