First-Class Mail & Postage | USPS
www.usps.com › ship › first-class-mailWeight & Shape Requirements Maximum weight for First-Class Mail letters is 3.5 oz; for large First-Class Mail envelopes and parcels, the maximum weight is 13 oz. All postcards and envelopes (or flats) must be rectangular, otherwise an additional charge may apply. Additional size restrictions apply depending on the type of mailpiece you're sending.
USPS Package Restrictions - EasyPost
www.easypost.com › usps-package-restrictionsMaximum weight is 3.5 oz. Flat: Min: 6 ⅛" x 11 ½" Max: 12" x 15" Maximum weight of 13 oz. Parcel: Maximum size is 108" in combined length and girth (distance around the thickest part). First-Class™ packages cannot exceed 22" x 18" x 15". Contents for Priority Mail Express, Priority Mail, or Media Mail® must weigh less than 70 lbs.