I've had the same issues with shipping little padded envelopes for small items. When I was told that I couldn't ship it because it wasn't thick enough, I took it back home and added some cardboard inside the envelope. It made it thick enough and I've done that ever since.
When I was told I couldn't add confirmation on the thin envelope, the postal worker had a piece of 6" X 8" cardboard with a 3/4" slot in it and slid my envelope through it. He explained that when it is thinner than 3/4" then it counts as a letter (which can't have confirmation). I actually ended up getting the same worker who tried it on the "slot" when I returned to mail the revised envelope and it was fine... with first class postage and confirmation.
I guess it was live and learn.
