I would argue that this story is a work of Realism rather than Regionalism or Naturalism.
In works of Regionalism, also known as Local Color, the setting is always of particular significance as is the culture and language of this setting. The setting of this story does not seem to hold any particular importance in terms of its characters or themes.
In works of Naturalism, nature is often revealed to be an incredibly powerful force that is apathetic toward humans. There is no higher power that watches over us, no God or Fate that runs the show. Instead, we are at the mercy of nature, which does what it does and goes about its business without any reference to us at all. There is no sense in this story that nature is the highest power; it seems, instead, that money or even human impulse may be.
This story is a work of Realism because it focuses on, without romanticizing, the very real and daily cares of a woman with several children and not enough money. Mrs. Sommers "knew the value of bargains" and "could stand for hours making her way inch by inch toward the desired object that was selling below cost." Mrs. Sommers is presented as a very human person: she has big plans and specific intentions, but she inadvertently does things other than what she intended. She treats herself for once in her life and becomes quite sympathetic in the process. Her anxieties and her humanity are presented in very realistic ways.