Just saw this the other day, thought it might be useful for people who're doing trades or FS:
The Priority Mail Small Flat Rate Box measures 8-5/8 x 5-3/8 x 1-5/8 inches — about the same size and shape as three stacked DVD cases.
It will have a retail price of $4.95 effective Jan. 18 — one price, regardless of weight, up to 70 lbs. for U.S. addresses. The international price, for up to 4 lbs., will be just $10.95 to Canada or Mexico and $12.95 to all other countries, with savings of 5 percent for customers who ship online. Online prices also are lower for shipping to U.S. addresses. The online price will be $4.80, and for large volume mailers who qualify for USPS Commercial Plus pricing, the price will be $4.75.