Oh, joy.
Had another customer (this one in California) report a FRE delivered postage due. I called her branch and spoke with the postmaster, who, being a truly nice guy, agreed to waive the postage so that all of us were happy.
He also gave me some info that sheds additional light on this issue.
Apparently a USPS ruling was issued on 12/28/10 stating that FREs must be no more than 3/4" thick.
And then a couple of days later, the ruling was reversed, saying that if it fits it ships. (Subject of course to the rules that already appear on the USPS website - regarding do not modify the envelope, must close with the adhesive being the means of closure, taping over the flap and seams are allowable.)
And my USPS Business Support Specialist reports another ruling; she says this is word for word what it says:
"Mail pieces presented for mailing in USPS produced Flat Rate packaging are NOT required to remain flat to be eligible for flat rate prices. The adhesive strip on the mail piece MUST be used to seal the flat rate packaging. However, while the glued flap must be able to seal the package before the addition of tape, the mail piece does not need to remain flat and the USE of TAPE IS permitted along the flaps and seams to reinforce the container provided the design of the container is not enlarged or reconstructed (e.g., opening the sides, extending the flap to make it seal, etc.)."