There are a lot of rules in Stamps. For example, you have to be super clean, even though the water is freezing cold. Also, adults are never addressed by first name, but by last name and one of a bunch of honorifics. Example: Dr. Shmoop.
Everyone knows these rules. Well, everyone except for those people Angelou calls "powhitetrash" (5.6)
When these folks enter the Store, they cause chaos and do everything they aren't supposed to, including calling Momma and Uncle Willie "Annie" and "Willie."
Marguerite tells us about the worst experience she has ever had with these people.
One day, after she was cleaning the yard, a bunch of white girls came to the Store.
Momma told Marguerite to go inside. We get the feeling this isn't going to end well.
The girls, rowdy and laughing, made fun of Momma while Marguerite fantasized about shooting them with a shotgun. Whoa there.
One of the girls did a headstand, and because she wasn't wearing underwear, flashed Momma her genitals. Momma didn't even blink. And so—finally—the girls left.
When Momma went back into the store, she found Marguerite crying tears of rage and told her to wash her face. She seemed like she had won some kind of victory Marguerite didn't quite understand.