Calculating shipping cost is a highly complicated thing. Each article has a weight and size stored in the system. At the checkout it calculates the total weight and volume of your order. Then it compares these data with a table which has all ZIP codes of New Zealand, to see if your address is rural or not, South or North Island, then it allocates depending on the courier provider the average cost for this kind of packaging.
It comes to a point where I just can't maintain all those combinations for endless amount of weight and volume. So If you order more than about 0.125 cbm which is >4 bags then the system won't find a courier. In this case just leave the shopping cart as it is and send me a message.
I can quote you the best freight cost for this order, sometimes also cheaper than what the system does.