5.0 Additional Physical Standards for Priority Mail
5.1 Physical Standards of Mailpieces
The maximum weight is 70 pounds. The combined length and girth of a piece (the length of its longest side plus the distance around its thickest part) may not exceed 108 inches. Lower size and weight standards apply for some APO and FPO mail subject to 703.2.0, Overseas Military Mail, and 703.4.0, Mail Sent by U.S. Armed Forces, and for Department of State mail subject to 703.3.0.
6.0 Additional Physical Standards for First-Class Mail
6.1 Maximum Weight and Size
Matter at First-Class Mail rates cannot exceed 13 ounces. First-Class Mail weighing more than 13 ounces is Priority Mail (123, Rates and Eligibility). The combined length and girth of a piece (the length of its longest side plus the distance around its thickest part) may not exceed 108 inches. Lower size or weight standards apply to mail claimed at certain rates or addressed to some APOs and FPOs subject to 703.2.0 and 703.4.0 and for Department of State mail subject to 703.3.0.
However:
3.3 Weight Standards for First-Class Mail Automation Letters and Cards
Maximum weight for First-Class Mail automation letters is 3.5 ounces (0.2188 pound). See 3.13.4 for pieces heavier than 3 ounces.
3.14 Enclosed Reply Cards and Envelopes
3.14.1 Basic Standard
All letter-size reply cards and envelopes (business reply mail (BRM), courtesy reply mail (CRM), and meter reply mail (MRM)) provided as enclosures in automation First-Class Mail, Periodicals, and Standard Mail and addressed for return to a domestic delivery address must meet the applicable standards for automation-compatible mail in 3.0. The mailer's signature on the postage statement certifies that this standard, and the standards listed below, have been met when the corresponding mail is presented to the USPS:
a. Each reply piece must include the appropriate facing identification mark (FIM) under 1.2, Physical Standards for Cards Claimed at Card Rates.
b. Each BRM piece must bear the correct BRM ZIP+4 barcode; each MRM and CRM piece must bear the correct barcode for the delivery address, subject to 202.5.0, Barcode Placement.
c. Each BRM piece must meet any applicable standard under 507.9.0; each MRM piece must meet any applicable standard under 604.4.0, Postage Meters and PC Postage Products ("Postage Evidencing Systems"); and each CRM piece as defined in 3.14.2 must meet the standards of this section.