Been awhile since I shipped anything internationally, but I happened to have two trading partners in a week that were outside the USA. Given prior experiences, I fully expected to pay at least $10 each to ship a regular 000 bubble mailer across the border, but I had some 6x8 rigid mailers left over, so I tried those, instead. I went ahead and filled out my customs forms (fretting over how to arrange French address details into the all too US-centric boxes, and when I got to the counter, the clerk said both of them should be fine as "large international envelopes", no customs form needed!
We'll see what happens, but I walked out of the post office having shipped a rigid mailer to Canada for under $4 and to France for under $7. I guess we'll see what happens!