I think someone came out with a memo or some crack down. I've been mailing for a few years now, same items, in flat white cardstock envelopes, and they went through fine. Last time I mailed on, it came back much like yours. I took it to the post office, the guy bent it, looked at it, measured it. And when I asked "how do I know it won't come back again" he took out a hand stamp and canceled it. I'll find out when the mail comes today.

When I wrote to USPS I got this back, which may be helpful?

I still think, that something changed in the detection or USPS has issued a directive with the new rates.