I don't know about that??
a jelly roll is 2 1/2" thick. my PO wont let me mail in the flat rate cardboard envelopes with it hoved up.has to be uniform thickness.
one PO wouldn't even let me ship in one if I reinforced the seal with tape.
I always err on the side of caution as once I had a package that arrived to the buyer with postage due because of this