If you're in the US and want to send a package of food to another country, do you have to declare it? And if so, what are the possible costs?

I'm asking this for my sister who's planning another trip to the United States, and she's thinking of sending some veggie stuff to her home address (which would be too heavy to take it back by plane). She said she ate some really delicious things during her last visit, and she would like to have some of it when she's back home.

Do you think that would work, or are there any restrictions?