Postal Service flats have a problem. As a group, they don’t cover their costs. Last year, they collectively lost about $630 million. First, what are flats? They are not European apartments or a type of shoe. In the postal context, “flats” are large mailpieces between the size of a typical letter and a thicker parcel. Full-size envelopes, magazines, and catalogs are all considered flats. It’s a very popular format.