While walking with Eliza, Mattie remembers two people from her past. Who are they and what does one "send" to her?

They were Mr. Peale and Nathaniel Benson.

Nathaniel Benson was sending her "flowers".  

Explanation:

The novel "Fever 1793" by Laurie Halse Anderson tells the story of Mattie Cook amidst the spread of yellow fever. The story follows Mattie through the hardships of running away from home, the death of her grandfather, yellow fever, and helping others.

While walking back from the orphan home with Eliza and Nell, Mattie remembers Mr. Peale, "the one who gave all those strange names to his poor children" and "Nathaniel Benson".

Nathaniel Benson was sending her "flowers" through the shutters, and had previously sent "a painting, a vase full of delicate flowers, bright blue, lavender, and red carefully painted on a scrap of wood."

nautre

nature is sure to be the best thing on earth

the sunshine and beauty is sure to be worth

this can't be sold

for any money or gold

because nature is strongly beautiful and is the best thing to happen on earth