In Walk Two Moons, why doesn't Sal send any postcards to her father?

The answer to this question comes in Chapter 10, “Huzza, Huzza,” when the travelers stop in Madison, Wisconsin. Gram asks Sal if she wants to send postcards to anyone, and the girl says she doesn’t. “Not even to your daddy?” she asks. Again, the answer is “No.” She can’t say it out loud, but postcards remind Sal too much of her mother. When her mother made her own trip from Kentucky to Idaho, she sent postcards home to her daughter at every stop. Sal remembers getting one from Mount Rushmore in the Badlands and one from Coeur d’Alene, Idaho. Her mother had written quick but loving notes on the backs of the cards. Sal wants to keep these exchanges special, between only the two of them. Sal’s memories cause us to continue to wonder what really happened to her mother; why she headed to Idaho in the first place; and why the three family members are now following her route, months later. The answers won’t become clear until much later in the book.

Why doesn't Sal want to send postcards to her father in "Walk Two Moons"?

Sal is angry with her father for two reasons:  one is the move to the city and the other is his friendship with Margaret Cadaver.  She is unable, until the end of the story, to understand her father's motives.  In addition to this anger, is the association Sal has with postcards.  Her mother sent her postcards, and it was the last contact Sal had with her.  The postcards were a reminder that her mother was gone and a symbol of Sal's feeling of abondonment.  Therefore, Sal herself has an aversion to postcards.