The Logans are encouraging their friends to boycott the local Wallace Store and shop in Vicksburg, where they're treated better. This puts them in a dangerous situation and even puts the land they own at risk.
Near the beginning of the book, Papa warns Cassie and the others not to go to the Wallace Store. He says that kids hanging around there are going to get in trouble one day; they buy liquor and smoke cigarettes after school. This is just after Mr. Avery also says that they don't want their families shopping there.
Not only is the Wallace Store an unsavory place, but they also discriminate against black customers and treat black people badly. That's why the Logan family makes the overnight trip to Vicksburg to shop. People there treat them better. They suggest that others do the same, but the trip and the lack of someone to sign for their credit makes them hesitate. Mama suggests that someone could bring them what they need and then says that they might be able to find someone to sign for them. This leads to a boycott that causes Papa to get shot and Mama to lose her job.