An order-picking method includes autonomously routing a plurality of mobile robotic units in an order fulfillment facility and picking articles to or putting articles from the robotic units in the order fulfillment facility. A material-handling robotic unit that is adapted for use in an order fulfillment facility includes an autonomous mobile vehicle base and a plurality of article receptacles positioned on the base. A visual indicator associated with the receptacle facilitates picking articles to or putting articles from the robotic unit.