Collapsible reusable carrying cases are provided in sizes varying from small food containers to large push cart bins on casters. The cases are assembled or disassembled from a joined flat space-saving configuration to a functioning case and vice-versa. All parts that make up a carrying case do not separate from the carrying case and no parts can be removed. The cases are formed from rigid plastic panels, and are assembled or disassembled without tools. The sides of the cases are erected progressively by hand by interlocking elements at the corners. The panels also disassemble progressively by being manually pulled apart. When folded into a flat storage or shipping configuration, the panels stay together, resisting unfolding by virtue of snaps which are either molded into the panels or bonded to them. The carrying cases are resistant to water, dirt, bacteria, molds, allergens, and inclement weather.