No Matter what style box ( I use a Plano compartment box) or envelope you use ( I use the padded ones) make sure your return address is clearly marked and affixed to your box. There have been a couple of times in my swapping career that the machines at the PO have torn the envelope and the box containing the flies fell out. Because I have a return address label on my box, both times I got my box and flies delivered to my door, courtesy of the USPS.

REE