I'm going to watch this thread - I've seen flat rate priority mail envelope tv commercials, even this morning! I've heard some are having problems and some aren't with filling the flat rate envelopes full - I've heard others say the PO won't take them if they are over 3/4" high..
I ordered the new bubble flat rate envelope mailers - I heard you can stuff those full - no matter how thick.. I also have ordered the new priority regional flat rate boxes - I haven't researched these rates yet - flat rate envelopes are the size I use the most.