I once visited in a tourism goods store my friend worked in. If you buy some paintings in such big specialized store for tourists, they will offer shipping service with perfect packing for you. The fare would be included in the paingting costs or perhaps not, you can ask clearly about that. If you buy those in cultural street or markets or a small painting stores, you can bring them to China Post and mail it back. Chinese painting rolls do not need too make space, or perhaps you would make some having been decorated. Better no shipping of the painting with glass. Some express company would provide better service than China post but charging more. UPS, FedExExpress all have branches in big cities. You may contact them.