If you use a cardboard box mate, make sure you line it with poly sheeting. This is to ensure that you have adequete insulation, and it adds extra strength to the box. Alternatively you can use a use a poly box, which if you have a local fruit and veg shop, you can obtain them for free from there, they use them to receive their broccoli and watercress in, and then they get put in the bin. Always worth a look.
If sending by Royal Mail, you must attach a couple of labels to the parcel indicating 'Live Fish Fry', I have sent fish 2" in length this way without any problems.
Hope this info helps.