Here is a summary of the changes next month in USPS postal rates. Notice your 20c stamp won't be enough for added ounces. I don't think they have a 21 cent stamp yet.

Highlights of the new single-piece First-Class Mail pricing, effective Jan. 26, 2014 include:
•Letters (1 oz.) — 3-cent increase to 49 cents
•Letters additional ounces — 1-cent increase to 21 cents
•Letters to all international destinations (1 oz.) — $1.15
•Postcards — 1-cent increase to 34 cents

Stamp prices have stayed consistent with the average annual rate of inflation of 4.2 percent since the Postal Service was formed in 1971.
Pricing for Standard Mail, Periodicals, Package Services and Extra Services also will be adjusted as part of a filing to the Postal Regulatory Commission (PRC) scheduled to take place Sept. 26.