Todd; I'm being told that the adhesive provided strip that comes on the flat rate envelope is intended to limit the thickness of the envelope ... that adding additional tape (even priority tape) allows the envelope to be expanded or wider than it was intended for the flat rate. I'm told that adding additional tape voids the flat rate and that if I want to add additional tape, then I must pay the regular priority postage in a non-flat rate mailer.

Hey, I'm just a dumb customer, doing what I'm told by the USPS employee here in my local post office in New Jersey. If you (or others) have a postal clerk or supervisor on your end that looks the other way ... be thankful.