The information I now have is -

The NZ$42 charged by Western Union does not include US$25
currency conversion and processing. That amount is deducted
from the money transferred by the receiving bank. So to transfer
US$190 to the merchant, US$215 has to be sent

The merchant can offer no other payment method - credit card for
example - so the cheaper option, by NZ$17, is through my bank
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