I tied flies commercially for many years & when shipping flies that I didn't have to be concerned about getting crushed, such as streamers,I placed them in a 4 mil plastic bag between cut pieces of card board, then into a padded mailer. The 4 mil plastic bags are tough & the extra card board kept the hooks from poking thru the padded envelope.
I used the Altoid cans too for dry flies & other flies that might get crushed, but otherwise a small plastic box inside a shipping box worked as well as anything.
One note about shipping plastic fly or other boxes, even inside a padded envelope they can get damaged since much of the sorting is done by machine. Plano or similar boxes are great, but do get crushed/broken sometimes. I came to prefer shipping in boxes with plenty of "padding" inside. It costs a little more but I had less problems.