The following explains the preparation of the paintings for the shipment; shown is an oil painting 29.9" x 29.9" (75 x 75cm).
The painting is wrapped in a plastic cover, placed between two hardboard pieces, and inserted into a cardboard cover. All is secured against any unwanted movement. Thus, the painting is prevented from being damaged even if the shipment is not handled properly.
Here you can see shipping 4 watercolors at once. The paintings are framed and the procedure is the same as with oil paintings, with a greater emphasis on the glass protection with a bubble wrap.
You can also have only the paintings sent and let them frame yourself to match your interior. Shipping paintings without a frame is much cheaper - postage for watercolors starts at 15 EUR.