It also helps if the inside is filled with packing material so that it is more difficult to collapse it under pressure. These little boxes often get mashed between the big boxes.

I was sent a box of chips by a friend in LV. Wen it arrived, it had been retaped and inside it had a music CD and a book! Go figure! It looks like a couple of them go crushed and the USPS folks just shoved a bunch of stuff into the nearest box & shipped it on. I much prefer the padded envelopes. Besides, Priority Mail doesn't buy you much time,anyway.