【How-to】How much is priority mail flat rate envelope

A mailpiece must be flat-size or larger, if rigid, or over 3/4″ thick to be classified and priced as a package (parcel). For padded bags (e.g., ReadyPost ^® “cushion mailer”), when the thickness is 3/4″ or less and the item is flat-size and somewhat flexible, the item should be classified and priced as a large envelope.