Sometimes a painting is a little bigger than the envelopes available, so I have been known to use a priority box at the post office without popping it out into box shape. I use the built in adhesive strips plus a little tape on the cuts for the folds. The USPS priority boxes come with a reasonable flat rate, insured, and tracked. I’m sticking that skinny label in my account book under shipping info/delivery date.
By the way, Phyllis, your ‘Cup Flowers’ watercolor is mailing out today.
This is how I wrapped Phyllis’ painting. I layered a stiff backboard (or foamcore), the painting, the mat, and the cardstock ad that comes with the mat (if I have one). I wrapped the layers in thin paper, like tissue paper. Then slid it into the flattened box. It arrived quickly and safely, and Phyllis thanks me so much more than I feel I deserve.
A large painting will require more structure because it will be easier to bend, drop, and lots of other containers will likely be pressing against it in the shipping vehicles. After wrapping the flat art in protective paper, I would sandwich it between foam core or cardboard layers. If the artwork is framed or dimensional, I found some great tips on shipping from Red Dot Blog here: http://www.reddotblog.com/wordpress/index.php/how-to-ship-paintings-a-step-by-step-guide-for-artists-and-galleries/