How to safely ship artwork
Unless you have insured your package, the US Postal Service really doesn't care whether your package arrives intact or not. If you didn't insure it, it's not their responsibility. Many is the time I've received artwork that was mailed in the flimsiest of paper envelopes, and many is the piece of artwork so mailed that was folded in half, creased, or torn. This does not have to happen. One can safely package artwork in a very simple, quick, inexpensive, and nearly indestructible manner.
The USPS will give you many different kinds of supplies, if you mail things using Priority Mail. Since the customer is usually the one paying the postage, there's no reason not to use Priority Mail. Just go to http://supplies.usps.gov/ and you can order all the shipping supplies you require.
To ship artwork safely, you will want to order items EP-14F (Priority Mail Flat Rate Envelope) and O-1092 (Priority Mail Box, 12.125" x 13.375" x 2.75"). With this, you can ship artwork up to 9x12. USPS will even deliver these items to your house or business for FREE!
Just place your artwork in the envelope and seal it. Then place the envelope in the Box, stuff lightly with newspaper (just enough to keep the envelope from bouncing around), and seal. Then just label it and ship! As long as it's less than 1 pound total, it still costs the same to use the box as it does to use a plain envelope! The box will be nearly impossible to fold or crease, and even if they do damage the box, since it's bigger than the envelope inside, the contents should remain safe.
And as far as labeling goes, if you use the USPS labeling software available from https://sss-web.usps.com (web-based) or http://www.uspswebtools.com/ShippingAssistant (Windows only download), you can get Delivery Confirmation for FREE!