Cadriel: They aren't always so specific. The last item I imported they only charged based on the item value - non-inclusive of the shipping cost. They were quite clear about doing it that way - and I got the distinctive impression that they 'felt sorry' for having to charge in the first place.
I can understand the GST charges - it bring's most items I import closer to the local price. However, the additional charges of $38 are a little excessive.
In most cases - if the item i'm importing would incur charges, I wouldn't bother importing. If the online store allows 'fiddling' with the declaration form - then I definitely import, as it becomes WAY cheaper.
Other things to be aware of; If you pay for the faster shipping options - don't expect that to make any difference when it arrives in NZ. They all go into the same queue - no matter if it's priority or not.
You should include some specifics of item values for the times when you are getting charged the fees so that it is clear what the circumstance are behind it please. Otherwise, for all we know you're importing a $2 empty box and being charge duty/fess/gst.
