Freight costs

Hi Claire,
Definitely, do not send the goods you buy in Nepal  in or with your baggage.  Doing so will cost you top dollar.
Let me hasten to say that although I am no longer active in this export business, I know how it works from A to Z in Nepal. You'll find many shipping companies that are active in this field in Kathmandu. Not all are good, but some are most reliable, and what you want to do is choose the better ones, people like Capital Export which I  used a lot in past years, but there are many others in the field.  The way it works is this. Freight rates are degressive. The more you ship, the less you pay per kilo. 
AIR Freight categories go something like (per kg rates are always quoted in USD):   
less than 45 kilos 
45kgs and more
100 kgs and more
I think you will find that rates quoted by the different shippers are pretty much all the same - not so much variation, really. But where there can be a difference will be in the quality of the service they render, the pick-up service they offer or do not offer free of charge, the packing, etc  All shippers will also prepare all the documentation, invoices, list/inventory of goods, etc. for you, but that needs to be done well. If done well, by a good and reliable agent you select, shipment of those goods to NZ will go all go smoothly, and when the goods arrive in your country, you won't have any difficulties at the NZ customs.
Shipping by air freight from Nepal to NZ should take no more than 2 weeks.
If you intend to ship a quantity of goods greater weighing more than 100 or 150 kgs, then you may wish to consider Shipping by SEA. Goods will be sent to Calcutta Port, India, via Birgunj in Nepal and from Birgunj to the indian port by road. Cost of shipping by sea costs a small fraction of air freight. But then the shipment by sea may take 1-2 months, depending. Shipping in what is called "part container" usually starts at about 250 kgs. The cheapest rates are of course for the bigger buyers who can fill either a 16 ft or a 20 ft container.  Hope this helps.  If you need a buying agent, I would recommend WholesaleNepal. Very good and reliable people I used to work with. I think they are still around in Kathmandu.