We had a USPS sales rep stop by on Tuesday, and I asked her to find out if really-stuffed envelopes are acceptable or not.
And... she researched the issue, and could not find any place where the rules say that the envelope has to be less than a certain thickness to qualify for the flat-rate price.
And you *can* tape over seams, and the flap closure, to reinforce those areas.
Thought those of you who are using these envelopes to ship your materials might like to know what the official word is.
Don't know how to advise you, though, if your own local Post Office is giving you grief about your envelopes. Other than to ask to talk to the PostMaster, armed ahead of time with all the definitions of what a Flat Rate Envelope is, printed off from their own website / Domestic Mail Manual (which is their "bible", so to speak).
It's been my experience that being a polite and knowledgeable customer, it's possible to ask the right questions so that they may reassess their position.