PS Form 3877, Firm Mailing Book for Accountable Mail is one of the forms provided by the United States Postal Service (USPS) to obtain a proof of mailing and to document extra services that are used by a mailer, but not reported in the mailing manifest. Manifest mailing is an automated system that enables a mailer to document postage and fees for all pieces in a mail paid via permit imprint indicia. Imprint indicia is one of the most popular ways to pay for postage using a postage block called "indicia", printed into each mail piece and a postage account from which the USPS deducts the total postage.
PS Form 3877 should be used for three or more mail pieces to be registered at the time. The form was last revised in January 2017 in order to remove references to international certificates of mailing service and is available for download below.
The form contains only one fillable page, the other two have a detailed guide for its completion. The mailer submits the forms in duplicate and receives the original copy as a mailing receipt after the entries are verified by the Postal Service employee who accepts the mail.