Comment by ErsatzVerkehr

The official USPS policy is that such mail is just thrown in the trash:

http://pe.usps.com/text/csr/PS-086.htm

Here is some other coverage:

http://consumerist.com/2007/11/01/update-taping-pre-paid-bus...

http://www.straightdope.com/columns/read/566/can-i-mail-a-br...