If you are on the business package for Shopify, you can use the real time carrier rate features to get shipping rates from major carries in real time. This is great since you do not have to worry about calculating costs manually yourself.
But we have see cases where this does not work well in some situation. For example, if you are doing drop shipping and you have products that can be shipped from a few different origins, Shopify is not going to know what and it will assume everything on the same cart will be shipped together. If you have special shipping rules such as an item exceeding a certain weight will have to be shipping individually, Shopify would not know that either. Or if you use multiple carriers and depending on the products, you will use different carrier, this is not supported either. Shopify has good reason to not support such cases as they need to provide a generalized solutions that work for most people most of the time.
In special cases like this, it is best to develop you own solutions that can perform custom calculation. We have experiences in this area, Talk to us and let us help you.
We are building a Shopify application to support getting New Zealand Post shipping rates based on customer requests.
The application allows you to offer New Zealand Post rate calculator with your Shopify store. It will present a list of shipping options and the corresponding shipping cost to your customers during checkout. Once customers select a shipping method, the shipping cost is added to the cart and customers can complete their purchases.
You will also have the ability to add a surcharge on top of New Zealand Post’s rate so you can recover your shipping cost.
If you are interested, I have set up a form where you can enter your contact information so that we can notify you when the application is ready. https://webifytechnology.com/shopify/new-zealand-post-shopify.php
Please let us know if you would like to see extra features not described here by posting a question in our support center.