Is it only relevant to US since Australia has Post codes not ZIP codes - obviously the same, but different thing according to different postal services. US code is like 90210, where Australian are 4 digits like 2000. Perhaps it has to do with Stripe (not Machform) so that payments are identified from whichever country they are coming from.
Suspect this is a Stripe thing, not a Machform thing.....try the test card number in your Stripe account sandbox mode, does it show the same thing - ZIP code following the CCV?