Well, you might not be overriding it, but they're defaulting you to a postcode.

First screenshot is if I specify my postcode as Mildura.
Second is the same (full price, out of stock) when I try 2000, 3000 and 6000.

Screenshot 2024-04-23 at 7.14.15 PM.jpg Screenshot 2024-04-23 at 7.15.04 PM.jpg

Try changing your postcode - see what happens... all in the same browser. It'll still remember a postcode change per session without cookies. I'm not using Firefox.