As of May 2015, the additional postage cost for a second ounce on a first-class letter or flat is 22 cents. For a first-class package, the second ounce fee is $2.04.
These fees apply in addition to the cost of mailing the first ounce. For a 1-ounce first-class letter, the cost of a stamp is $0.485 as of May 2015. The 1-ounce flat is 98 cents. A package weighing 1 ounce mails for $2.04, the fee per ounce for the first three ounces.
Businesses sending automation first-class mail can qualify to mail the second ounce for no additional charge provided certain criteria are met. For instance, in addition to meeting all regulations applying to regular first-class mail, exempt pieces must be part of a single bulk mailing of 500 automation pieces.