I used up some of those (**& plastic bags for packing in Xmas presents that I shipped. I filled them about 1/2 full of styrofoam peanuts. amd tied them loosely. Since the bag isn’t full, it molds nicely around the various odd shapes in the box, and the styrofoam stays corraled instead of sticking to everything. Better than BUYING bags of air!
You can also fill the bags partly full of popped popcorn, this also molds well and protects, is cheap, is environmentally friendly and has the added bonus of being an extra gift for the recipient…