There was another web thread about this, and on that other thread someone said they make a second flat rate mailer that isn't paper, but it can only be found online. The post office is finding that people are stuffing the paper ones to maximum capacity, and they are ripping in transit. The new one that isn't paper is more sturdy, and still has that same rate and old terms of use, or did at that time of that post. The new regs may supercede that- but I agree that they better change their motto of "if it fits, it ships"!
I had a package mailed to me that had 5 yards of fabric, but it ripped partially in transit, and only the inner plastic bag held all the fabric in.