USPS Press Release:

WASHINGTON, DC — A new Postal Service rule goes into effect next week for packages and envelopes that weigh more than 13 ounces, if they’re being mailed with only stamps as postage at a location other than a Post Office retail service counter.

Starting Monday, July 30, customers can use one of several convenient online postage applications — available 24/7 — or an Automated Postal Center, if they wish to mail items that weigh more than 13 ounces in Postal Service collection boxes or Post Office lobby mail slots; or if they wish to leave the items for pickup by their letter carriers. Online postage applications include the Postal Service’s Click-N-Ship service on usps.com and PC Postage from an authorized USPS vendor.

If a customer is unable to use one of the above methods to prepare and affix postage, items weighing more than 13 ounces must be presented for mailing to an employee at a Post Office retail service counter. Business customers who use postage meters may continue to use meter postage for packages of any weight and mailing method.

Customers will notice new decals on USPS collection boxes, and Post Office lobby and Automated Postal Center mail drop slots. The new red, white and blue decals inform customers that deposit of stamped mail over 13 ounces is prohibited, and any such mail will be returned.

Previously, the prohibition applied to mail over 16 ounces. The change is part of ongoing security measures established by the Postal Service, in cooperation with other government agencies to keep the public, customers, employees and the U.S. Mail safe.