Here's what you need:
1. Two (2) Priority or Express Mail boxes (15.5" x 21.5") from your local US post office. These are free.
2. Packaging tape
3. Box cutter
Step 1: Step One: the Interior Protective Envelope
Start by turning the box so that the short side is towards you.
1. In the lower left of one of the boxes, measure and pencil out a 13.5 x 19.5 inch box.
2. Cut along the 13.5" side, removing and discarding the entire 15.5" x 2" portion.
3. On the 19.5" side, mark out a 7.5" x 2" rectangle in the top right, and another in the bottom right.
4. Cut out those 7.5" x 2" rectangles, leaving a 4.5" x 2" tongue in the center.
Step 2: Step Two: Complete the Tongue Flap
2. Cut the top side of the tongue along the 19.5" line. Do not cut all the way through, or you'll just be cutting the whole thing off.
3. Unfold the tongue flap. It will be attached to the bottom side.
4. Next, let's make a slit where the flap will folded over and inserted. Cut out a 1/8" x 4.75" slit about 2.5" inches in from the base of the flap.
5. Place your matted artwork inside the envelope, and close it by inserting the flap.
Step 3: Step Three: Construct the Mailing Envelope
1. Take the second flat mailing box and find the overlapping 21.5" edge where the ends were glued together. In the Express Mail box below, this edge is where the thick blue line is near the bottom.
2. Insert a cutting mat or extra piece of cardboard inside, and cut along and through that edge on the top side.
3. Unfold the box and turn it inside out so that the blank, brown interior is now on the outside.
4. Place the interior envelope inside.
Step 4: Step Four: Seal It Up!
2. Using the packaging tape, seal the sides.
Now just address it and put the usual extra warnings on like "FRAGILE" and "DO NOT BEND". You're ready to mail your poster!