Where does it say AMAZON or NZ CUSTOMS can 'do whatever they see fit' (richms) or 'use any value they like really on imports' like you say?
Thats sounds like hogs-wash... they are not a law unto themselves, surely they must abide by the facts as evidenced/shown?
"Customs has the authority to amend the declared value of the goods if it has reason to believe it is incorrect."
"An Import Entry Transaction Fee (IETF) of NZ$29.26 (GST inclusive) is charged to risk assess and process imported goods. An MPI biosecurity system entry levy of $19.98 (GST inclusive) is also payable when an IETF is charged, making the total fees payable of $49.24 (GST inclusive)."
"There is a common misconception that items purchased from overseas websites for less than NZ$400 can come into the country (be imported) without incurring any duty, GST, or charges. This ‘rule of thumb’ does not apply to items that attract both duty and GST, such as clothing, shoes, and accessories. Customs charges may be payable when the value of these exceeds approximately NZ$225."
Amazon is playing safe. It charges you a DEPOSIT. If Customs collect nothing or less you get a refund. If Customs collects more Amazon pays the difference and you're better off.
As I said before, too many people think they know better but if Amazon is doing it is probably because they want to keep on the good side of the law and continue to being able to send parcels here. IF they start being dodgy then Customs will stop the service and make YOUR life hell.