This article is from the eBay FAQ, by William B. Shaw toppsoft@yahoo.com with numerous contributions by others.
The USPS provides certain Express Mail, Priority Mail, Global Express
Guaranteed, Global Express Mail and Global Priority MailĀ® packing materials
at no cost to the consumer specifically for use in shipping via these
services. For information on usage restrictions:
http://supplies.usps.gov/disclaimer.htm. No one knows how many federal laws
you would be breaking by misusing those supplies and the postal inspector
probably won't be knocking on your door (and then, they might . . .), but
you are certain to get hassled by your local postal employees if they catch
on.
 
Continue to: