Dave,
I got the email about the problems with the Oz shipping. I thought the envelope self gluing might not hold. plus taping them together doubled the mass (like a battering ram). If you tape anything I would tape the flap down on the corners or the edges, or better yet a thicker padded envelope.
Oh, the US postal machines are quite brutal. More than once I've gotten a letter back with pieces of a my original mail in the letter from the "machine ate it for lunch dept." or something like that. The letter I sent looked like you taped it to the underside of car tire and drove across country. not much left of it. The remains of what they dug out of the machine's gears.
thanks.