Net mass would be the mass of the object in the carton, without including the mass of any of the package. For purposes of law and commerce, weight and mass are considered the same thing, so net weight = net mass, in this case. From a scientific standpoint, they are not the same. Weight is the force on an object due to gravity, while mass measures the force needed to change an object's speed or direction. Weight changes as you move away from the earth, but mass remains the same on Earth or in space (at lower, non-Einsteinian velocities, of course).