Respective items are retained as they are transported by mobile delivery mechanisms. An item to be delivered to a target storage location of a plurality of storage locations is positioned upon the carrying surface of a delivery mechanism. The carrying surface defines a retention zone bounded by a first wall, a second wall, a third wall and a fourth wall. At least one of the third wall or the fourth wall are displaceable to define an opening in the retention zone. The delivery mechanism moves from the location where an item is received to a location proximate the target storage location to facilitate transfer of the item through the opening obtained by displacing one of the third wall or the fourth wall. Irregularly shaped items are free—during movement of the delivery mechanism—to move upon the carrying surface and within the boundaries of the retention zone.