How about using a small box or like a check book size?
Good idea but not useful if the custom officers in your country or the receiver's country have the right to check whats inside your box.
You can follow what I said earlier because in most of the cases they go unnoticed. Make sure the envelope isn't too thick. Use the largest denomination of bill possible. ($500 = 5 x $100 bill etc) Up to 8 bills will fit into a #10 envelope along with a couple sheets of printer paper.
No custom officers will open any envelope. But they might open a small box if they feel something suspicious inside.
