A substantially planar shipping container blank is configured for planar stacking to permit compact packaging but simplifies the ease of assembly into a triangular shipping container. The container blank is foldably divided into a rectangular longitudinal section, two end sections, and a closure tab. The interior surfaces of certain portions of the end sections are pre-mounted to the interior surface of the longitudinal section. By folding the longitudinal section of the blank along two pre-pressed longitudinal fold lines, the end sections of the blank automatically fold along pre-pressed angular fold lines to form a triangular shipping container. The container can thereafter be fastened shut by folding the closure tab along a third pre-pressed longitudinal fold line and affixing the tab to the exterior of the container.