I believe that your question refers to the package that Mattie receives in chapter 10. This is a dire time for Mattie. Her mother has contracted the fever and is being bled by Dr. Kerr. Her grandfather has insisted that Mattie leave with him for the countryside for their own safety. Although she desperately wants to stay with her mother, Mattie has no choice but to get ready to leave the following morning.

As she is preparing to leave the city, Eliza gives Mattie a small flat package wrapped in brown paper from Nathaniel Benson that was left by the front door. This package contains a beautiful still-life painting depicting a vase and multicolored flowers. Along with the painting is a note that explains that Master Peale has closed up his shop and home where he, along with his family and all his assistants, will hide from the disease. Nathaniel promises that when the disease has passed, they will get together again to watch Blanchard's hot-air balloon be launched.

This shows that Nathaniel is still keeping Mattie in his thoughts despite the ongoing crisis. Despite these feelings, Nathaniel is stuck inside the Peale home, meaning he can't be of any actual help to Mattie.