envelope of sugar

"envelope" と言うと入っている中身はつい手紙や書類と思いますが、Stephen KingのThe Running Manを読んでいたら、こんな物を入れるにも "envelope" が使われのだと知りました。
They were let into a cafeteria where they showed their I.D. cards again. Richards took a tray pushed it down a stainless steel ledge. He was given a box of cornflakes, a greasy dish of home fries, a scoop of scrambled eggs, a piece of toast as cold and hard as a marble gravestone, a halfpint of milk, a cup of muddy coffee (no cream), an envelope of sugar, an envelope of salt, and a pat of fake butter on a tiny square of oily paper.