This is a good question. There are two salted plum incidents in the story. It comes right after the advice about "the art of invisible strength."
In the first mention, Waverly and her mother were in the store. Waverly wanted salted plums, and she threw a tantrum. When she threw that tantrum, her mother told her to bite back her tongue, and she did not buy her the bag of salted plums. There is no explanation of why Waverly's mother acted in this way. However, it seems reasonable to say that she acted in this way to discipline Waverly, to teach her the importance of controlling herself.
In the second incident, Waverly and her mother were in the same store, but this time, Waverly's mother bought her the plums, because Waverly did not throw a tantrum. It seems that Waverly learned her lesson to control herself. When her mother saw this, she rewarded her with the plums. Here is the quote:
"Bite back your tongue," scolded my mother when I cried loudly, yanking her hand toward the store that sold bags of salted plums. At home, she said, "Wise guy, he not go against wind.
The next week I bit back my tongue as we entered the store with the forbidden candies. When my mother finished her shopping, she quietly plucked a small bag of plums from the rack and put it on the counter with the rest of the items