Final answer:
Jeannette discovers her parents' move to New York indirectly through a letter that informs her of significant news, prompting her to make personal decisions about her own future.
Step-by-step explanation:
Jeannette learns that her parents have moved to New York City through a letter she receives while working at the modiste's establishment. The content of the letter or the realization it prompted is not explicitly mentioned in the excerpts provided, but we can infer from the context that the news of her parents' relocation is what spurs Amy to consider her own future, to value her independence, and to envision setting up her own establishment. The letter prompts her to immediate action, as she decides to leave her current job and life in New York behind.