Packing peanuts are the extra chicks added to a box, to keep the ones you ordered warm. They are different than the rare free chick that you get.
Years ago, when I ordered my first chicks from MM, I got packing peanuts. I was young and what I didn't realize, was that not only is there a 25 chick minimum, you need to order in multiples of 25. Otherwise, they fill in the rest of the last box with enough extra chicks to equal 25. This is to keep the chicks warm enough on the trip. The boxes are sized for 25 chicks and they all give off a certain amount of body heat in that space.
Usually, people get the extra cockerels from whatever hybrid layers they have. These are bred to be easy to sex as soon as they hatched, by just looking at their feathers. Ideal allows you to have a minimum order by price, instead of the number of chicks. They also include extra chicks for warmth, depending on the number ordered and the time of year.