An Unmanned Aerial Vehicle (UAV) can carry packages to locations that are unreachable by a ground vehicle using a gripper that can handle standard sized packages. This paper presents a UAV-mounted system for grasping corrugated boxes. The proposed gripping system uses two pairs of end effectors which are arranged perpendicular to each other. All four grippers are geared so that they can be actuated by a single control input. The mechanism is designed to handle position and orientation errors of the box relative to the gripper. The gripper was experimentally validated using a test box that was misaligned in position and orientation.