What I have done with watercolour paintings is to cover the painting with tissue paper, place it into a plastic cover or bag, then sandwich it between two mat boards, then place it all into a bubble envelope. Of course, other types of boards could be used (like sign boards). The boards help to prevent any crushing/bending etc. I also place a sticker on the envelope not to bend or fold or staple…….. Of course, this was for 1/4 sheet paintings, and smaller. I’m not sure what one would do with a larger painting. Hope this helps.