nzkiwiman:

 

I always let Amazon do the conversion for me. That way I know exactly what I am paying when the order ships in a few days, and I don't have to worry about extra fees by the bank
I also have some another form of proof of value in case customs jumps in and wants their cut

 

Customs calculate the value based on the foreign currency rate using their own published exchange rates which are NOT updated daily. They do not calculate this on what you've paid.

In a nutshell you're actually just overpaying for goods using the Amazon conversion because the converted rate incl bank fees will be cheaper than the Amazon dynamic currency conversion, and if you are very close to the import threshold for GST/Duty/IETF this will offer you no protection if the exchange rate customs use isn't in your favour.