not in this swap, just nosing in, cause i've been watching y'all (missed my chance to join, living vicariously)..... if you're sending a stretched canvas, put a sheet of newsprint, tracing paper, or plastic on either side (sticking it in a garbage bag will work), then several layers of thick cardboard or a sheet of foam on each side, sandwich-style.... then if you really wanna be safe a layer of bubble wrap around the whole thing, then slip it all into a box cut to fit snug. it's mighty tempting to put it in a big envelope, but you'll regret it when some dumba** at the post office sets a box on it and punches a hole in your canvas.
OR... if you used a flexible paint, the easiest, safest, cheapest way to go is to turn the picture over, use a butter knife or flat-head screwdriver to gently pry the staples out, and remove the canvas from the stretchers. lay plastic or newsprint down, then lay the painting on it face down. if you're careful, you should be able to disassemble the frame itself. if you don't want to, or you break the frame, you can buy to-be-assembled stretcher bars at a hobby store for a couple bucks a piece. stack your bars together, and wrap them in a layer of bubble wrap. then set them at one end of the painting and use them as a roller to roll the painting (and plastic) up on! wrap the whole affair in a couple layers of bubble wrap, then in either some stiff brown paper or drop 'em into a mailing tube! if the receiver doesn't wanna re-assemble it themselves (pretty easy to do.... if you have a pair of pliers and a staple gun) they can take it to a hobby store in their neighborhood and get it stretched for probably under ten bucks. I know it all sounds a bit finicky, but safe is better! i used to work in the registrar's office in a museum in manhattan - packing and shipping artwork was one of my main jobs. if anybody else has questions or wants me to explain how to re-stretch a canvas, gimme a shout!