Which of the following is typically classified as a flat mail-piece?

The classification of flat mail refers to mail that is larger than standard letters but doesn't fit the definition of parcels. Typically, flat mail pieces include items that are rectangular, are no thicker than 3/4 inch, and can be easily stacked. Magazines fit this description as they are larger, often have multiple pages, and maintain a standard rectangular shape. They are designed to be sent without a protective envelope, which further categorizes them as flat mail.

Other choices, while they might seem similar, do not fit the flat mail classification as precisely as magazines do. First-Class letters and postcards are categorized as letter mail due to their smaller size and thickness. Parcels, on the other hand, are larger and are usually packaged in boxes or padded envelopes, which means they exceed the dimensions for flat mail. Thus, magazines are the quintessential example of flat mail in this context.