Hi Sam,
I send fish and eggs via Royal Mail, (what they don't know, wont hurt them). What you need is a strong poly-box. (I get mine from the hospital, dims. are around 12"x12"x8" deep) line the box with bubblewrap or newspaper, Net your fish and place one fish in each bag with 1/3 water and 2/3 air. Now double bag this, the reason being is that if one bag leaks then the other will provide a sort of puddle and the fish is more likely to survive than if the water drains completely. Clearly label the bags with species name etc. And seal the poly lid with heavy duty tape. Wrap the whole thing in heavy brown packaging paper and address and label Fragile. You can add a heat pack inside of the lid if you think that the weather is going to be cooler, especially at night. Sent first class via Royal Mail the package should reach the recipient the next day. As for the cost, well that depends on the package, mine usually costs around £3.50 and I look to make £5.00 clear per pair of Killi's.
I hope this was of some help to you.
Regards
BigC