Do you mean couriers to send them through, or how to prepare and pack them ready for posting?
If you mean how do. Make sure he doesn't feed them 24 hours prior to posting, as this will reduce waste given off by your fish whilst in transport. Make sure you bag them in 1 part water, 2 parts air. Then you need a cardboard box. When you have that, put polystyrene round the sides, and start packing with newspaper. Then, place the bag(s) in the centre of the box (best to double bag them in case of leaks) and once they are in the box, continue to add more newspaper. Do this until the bags are tightly packed in the box, to stop movement while in transport.