have one of those brass-like, double-pronged clasp? (Poke through the hole and fold each prong over.)

I have been told by my Post Office that using those envelopes (with the clasp present) automatically bumps the item up to another category because with the clasp, it should not be processed through a sorter!

I am a bit fuzzy on this, because I did not challenge and just removed the clasp when ever I folder-over a large envelope to look like a letter envelope!

Jim