It is inaccurate to refer to this excellent short story as an allegory. An allegory is a literary work in which characters, settings, and events stand for abstract ideas or moral qualities. Allegorical characters are often one-dimensional. It is clear that there is no sense in which Mrs. Sommers can be described as a one dimensional character. However, I do think it is possible to draw a moral lesson from this story about giving in to temptation. There is definitely an allusion in the tale that is very reminiscent of another example of temptation, which is when Mrs. Sommers feels the silk stockings:
But she went on feeling the soft, sheeny luxurious things--with both hands now, holding them up to see them glisten, and to feel them glide serpentlike through her fingers.
Note how the stockings are compared to a snake, which is a renowned symbol for temptation. This is significant because the silk stockings are the first temptation that Mrs. Sommers gives into. After that, she easily carries on spending the money on selfish indulgences. Thus it is possible to say that the moral message of the story is the dangers of giving in to temptation and how, once you have started, it is very difficult to stop.