An excellent question Richard! Here is a post that describes some aspects of our practices for shipping painting and drawings from the studio to a gallery or frame shop:
http://www.smartermarx.com/t/packing-a-drawing-or-painting-for-shipping/161
Many here ship unframed drawings with something like this:
Here’s another useful insight:
https://www.outdoorpainter.com/how-do-the-pros-ship-paintings/