Is it a Postcard, a Letter, a Flat or a Parcel? Periodical Class nonmachinable flats ). The Postal Service uses the word " flats " to refer to large envelopes, newsletters, and magazines. The captured mail is held out for either immediate pickup by the caller or reshipment by USPS via Priority Mail. Flat definition, horizontally level: a flat roof. DMM Revision: Centralization of Bundling Standards.
Priority Mail, including flat rate shipping products, is an affordable and fast service from USPS. What made you want to look up tidal flat? Please tell us where you read or heard it (including the quote, if possible).