What does it mean to send charcoal in the snow?

In the snow, copper coins are the keepsake of little clay figurine Jiang Ni's yearning for the prince. At the back of the book, there are often descriptions about the little clay figurine's obsession with copper coins, including the little clay figurine counting copper coins in the palace during the restoration of the Great Chu Dynasty; When Chu was dying, when the prince took the little clay figurine away, Jiang Nilin wanted to get the copper coins back when she left. Later, when they were on Wudang Mountain and the prince drew lots for fortune telling to earn copper coins for Jiang Ni, she also smiled.