The January 22, 2012, price changes will include a major new benefit for businesses and organizations sending First-Class Mail® Presort and Automation letters. As part of the new Domestic Products prices being implemented on that date, First-Class Mail Presort and Automation letters weighing up to 2 ounces will be charged the 1-ounce price.

2nd Ounce Free is one of the many ways USPS® is add­ing value to First-Class Mail service. Commercial mailers can use the free second ounce to enclose promotional materials, advertising, coupons, and other messages that put them in touch with their customers and which could turn costs into potential revenues.

2nd Ounce Free is not a promotion, but a permanent price change for First-Class Mail Automation and Presort letters. An earlier initiative, the Reply Rides Free/2nd Ounce program, ended on December 31, 2011.

Source: USPS

http://about.usps.com/postal-bulletin/2012/pb22328/html/info_002.htm#ep1440733