Which of the following best describes a flat-sized package?

A flat-sized package is best characterized by its rectangular shape and compliance with specific thickness requirements. Typically, flat packages must be uniform in size and shape to facilitate efficient handling and processing within postal systems. They often fall under certain dimensional guidelines that define their eligibility as a flat, including maximum and minimum dimensions related to length, height, and thickness.

While other options might touch on aspects like thickness and size, they don't encapsulate the complete definition as effectively. For example, just stating thickness alone may overlook the necessity of the rectangular shape, while sizing limits—such as those mentioned regarding length—don’t capture the essence of what makes a flat package distinct. Thus, identifying it as a rectangular-shaped package meeting thickness requirements provides a clear and precise understanding of what defines a flat-sized package in the USPS context.