After the parchment or tracing paper against the painting, I place cardboard top and bottom, then I place (small works) in a bubble wrap envelope. A bit larger goes in a box. Once the cardboard protects it you can use bubble wrap or any padding you want.
If a painting is very wet I say in shipping that it will be ready to ship on xx/xx/xx date. When purchased I restate that in an email to the buyer.