Rear stays aren't very strong when they don't have a wheel between them. I would want to stick an old hub or some kind of block in there is shipping a frame stripped down. When its buried in a duffel bag, no one knows that its a bike frame.Forks aren't either so it's best to place a spacer in both the fork and rear dropouts no matter how you pack the bike for shipment. Bikes shipped to bike shops have plastic fork spacers installed for shipment and you can get them free from any bike shop.
For routine shipping use, I've made reusable spacers from defunct hubs. Your bike shop shop should be able to provide these at no or minimal cost. Mine were free for the asking.
I removed the axles, cones and locknuts from the hub shell, threaded the cones back on and then add the locknuts to the bare axle. Adjust the width between the locknuts to be a snug fit in the dropouts and tighten the cones against them to keep them in place. Install these in the dropouts and use your qr skewers or, if you used threaded hubs, use the axle nuts to keep them secure.