It’s not Royal Mail who are imposing the charges it’s customs and excise. So it doesn’t matter who the courier ir postal service is if they decide your paying that’s it I’m afraid. The charges are paid by the postal service though and you have to pay them back.
There is an agreement between UK and New Zealand called an MoU - Memoranda of Understanding. This is where the seller can pay the import Vat prior to despatch and prevents a handling charge by Royal Mail. Not much comfort