I have shipped an 18x24". The framer I worked with recommended our UPS store. They did a great job, charge was reasonable, and the package arrived in tact.
That being said, when I get sheet glass shipments (if I'm not ordering enough to require liftgate/freight), they are secured between large sheets of rigid cardboard and the cardboard is floated in peanuts. I would probably do cardboard, bubblewrap, and then peanuts.
Oh, and insurance