I think China EMS remains to be the popular choice, since the delivery can be tracked online and a lot of people have already used it. Reviews about its performance varies, but to help you understand how it works, I found this post very useful - Link hidden: Login to view
There are some suppliers who have a business account with shipping companies. They usually receive discounted rates, so it would be better to ask from the suppliers and take advantage of these partnerships.
Try also to first check the service that these shipping companies are sending it by - they sometimes use an express service when you could ask them to ship by normal air and have the item delivered at a more reasonable price.
There are also companies that provide rate comparison between shipping services, and will act as a middleman for you. I found this one site but I do not recommend it, for being online just last year. However you may use their online calculator to compare rates across different providers, from China to New Zealand - Link hidden: Login to view
Hope this helps. :)