Don't know if anyone else has encountered this, but I thought I'd share. (A2 size)
After using my new envelope punch board, I went to the P.O. to mail a few things, including the card sportin' my envie.
I didn't know whether to laugh or be annoyed when they tried to charge me extra for that envelope (contents were flat). When I asked why the upcharge, the employee told me that "square envelopes cost extra." (that I knew already). I pointed out that it really isn't square, just close; she measured to her satisfaction, harumphed, and charged me regular, first-class postage.
So, guess the moral of the story is that, if you're making A2 size envies, be sure the P.O. knows they are NOT exactly square, and to charge accordingly.