This can be complex but it doesn’t have to be.
My paintings are all on stretcher or plywood pannel so they need to be boxed. The painting is always fully insured when shipping. I make sure that the painting can be tracked so that I know where the shipment is at all times.
There are a few companies in my area that specialise in shipping art. I’ve included their URLs below. They come to my door, box the art and then ship it. This is how museums ship their paintings.