I think the place to start is to ask yourself what might happen in shipping and how to mitigate.
It could get wet – I put the art in a sealed bag
It could get bent – I don’t use just cardboard if I can avoid it. I like Masonite as it is durable and cheap
It could get cut/sliced – I try to use oversized boards/Masonite so that the art is not near where it might get cut out of a package.
It could break – I have heard that Masonite can break/shatter. While I have not personally heard or seen this, putting a layer of cardboard between the art and Masonite should keep any shattered Masonite from getting to the art.
It could get lost – always use priority and tracking
So, art in plastic … plastic taped to oversized carboard … cardboard in between Masonite … and to be overly secure, Masonite sandwich in a box with packing peanuts. I have heard foam core is a good substitute for Masonite but have not tried it.
I don’t ship a lot of art and don’t always follow this method as I don’t always have all of these materials but when I ship other people’s art that I have picked up at a show, etc, I go all out to make sure it arrives safe.