Find a company who knows how to use a UPS flat rate box...
A Tyvek envelope costs me over a dollar.
A small box a buck and a half.
And larger ones cost more. Look at uline. This crap costs. And the packing materials inside the boxes will eat you alive... Bubble, peanuts, whatever - it ain't free.
If you sell bullets, the USPS flat rate box is your friend.