Mrs. Sommers's shopping spree seems to be motivated, at first, by a desire for something beautiful, to have something just for herself. She initially considers what would be the "proper and judicious" use of her funds, which seems to be the choice she must always make (with several children at home); however, she is lured by the temptation of the silk stockings. Her first purchase comes about as a result of the luxurious feel of something that Mrs. Sommers has not had in a long time. Moreover, they're on sale and there are several pairs in her size, and this makes it easier for her to take the first step.
Further, the fact that her neighbors sometimes talk of the "'better days'" Mrs. Sommers had before she became Mrs. Sommers helps us to understand that she has not always had to deny herself, that the specter of the "gaunt monster" of the future (which we can read to be poverty) is something to which she's had to become accustomed. Her spree, then, also seems motivated by a need to escape her present situation, an interpretation that is supported by the fact that Mrs. Sommers goes to a theater matinee (an escape from reality) and that she half wishes that the cable car she takes home would just keep going forever. She seems exhausted by the constant need to be careful -- to be judicious with her money at all times -- and she wants a day where she does not have to count her pennies and think of her children's needs first. In other words, Mrs. Sommers also wants to be selfish, just for a while.