The USPS current regulation reads (emphasis ours):
Under the revised standards, domestic and international mailpieces that weigh more than 10 ounces or measure more than one half-inch in thickness, and bear only postage stamps as the postage payment method, may not be deposited into collection receptacles, including street, lobby, and apartment boxes, or other unattended locations.
These stamped mailpieces also may not be picked up by city, rural, or highway contract letter carriers for delivery, either from an individual or through Pickup on Demand® service. Instead, a mailer must present such a mailpiece to an employee at a Post Office™ retail service counter.
These restrictions do not apply to any mailpiece that weighs 10 ounces or less and measures one half-inch or less in thickness, nor do they affect any mailpieces, regardless of weight or thickness, for which postage is paid with a method other than stamps, such as a permit imprint or a postage evidencing system (a meter or PC Postage® product).
This means that:
YOU DO HAVE TO MAIL FOM THE PO and hand your package to a PO employee if
YOU DO NOT HAVE TO MAIL FROM THE PO
OR
*the limit used to be 13 ounces - it changed October 1 2019 to 10 ounces and 1/2 inch thick. Read more about this in the postal bulletin re: DMM changes here: https://about.usps.com/postal-bulletin/2019/pb22530/html/updt_001.htm
These regulations regarding stamped packages over a certain weight are designed to prevent untraceable packages that could contain dangerous material from being sent via USPS. They were originally put into place after the Unabomber sent bombs through the US Mail, and they have been refined several times since then. The weight limit was changed from 16 ounces to 13 ounces on July 30, 2007, and then to 10 ounces and 1/2 inch thick on October 1, 2019.
Related links:
step-by-step directions on how to wrap a book in only minutes!
How to Print the PBS Wrapper
How to wrap a Mass-Market Paperback book
How much does it cost to mail a book?
How to send a book
Is the postage amount shown on the PBS wrapper always correct?
The postage on my wrapper is/was wrong!
Does the postage print out on the wrapper?
I forgot the postage!
Using an APC for mailing books