For domestic and international mailing rates, USPS.com has the most up-to-date rate charts. Any recent changes are listed, as well as retail and commercial pricing for Priority Mail, First class, Express and many other mailing options.
Another site for quick reference to the latest postage rates and changes is PitneyBowes.com. Pitney Bowes is a global technology company that makes postage meters for business use. The highlighted rate changes for May 31, 2015 show an increase in Certified Mail rates, postcard rates and the charge for additional ounces for letters, flats and parcels. The regular domestic rate for a one-ounce letter or flat stayed the same.