What is USPS - Certificate of Mailing?

[This post is written and copyrighted by FIRE Finance (http://firefinance.blogspot.com).]Nov 25, 2009: We all know about USPS - Certified Mail but did you folks know there is something called Certificate of Mailing? We didn't. Recently we had to mail a $10 manufacturer's rebate claim. Many of the rebate processing centers try to disqualify a claim at the slightest opportunity. So keeping photo