I use NZ Couriers - within the NI they charge $19.50 for cash sales. You will also need to purchase or obtain a polybox or a solid container, and a heat pack. I purchase courier tickets which are discounted at $12.50 each I think but even then by the time I purchase a polybox and heatpack it still adds up. The price depends on the size box you use also and the cheaper rate is normally for a box less than .25 cubic metres.
Selling bulk fish makes it worth it and often I carry part of the cost myself for multiple purchases.
Not hard to do, if you arrive at the counter with the fish boxed they will take it, I label mine clearly and never a problem. If you ring and ask will they take it the person on the end will usually say no.