Bikes are send very safe in a big box (usually 120x60x60 cm with DHL).
We send bikes partial dismanteled to fit packet box sizes. The wheels, the right side crankarm and pedals are removed. The rear derailleur is screwed off and taped to the inside of the rear triangle without loosen the shifting cable. You simply can screw it in again and shifting will work without the need to adjust. We put in a transport axle instead of the wheels. The frame is wrapped into pop film. Wheels and parts are seperated from the frame in the box. The complete box is filled with padding material to avoid parts touching each other or damages from outer side.