Packing peanuts are always put in the trash, and therefore dumped into the landfill. Sitting in a large pile slowly decomposing over a million years. Which is a problem because they take up a lot of volume for their weight, which equals the landfills overflowing faster
. Instead you can drop them off at your nearest packaging store (UPS) and they'll accept them. Also you can go to a packaging peanut reuser.
Find a reuser here: http://loosefillpackaging.com/