So after much exploring the only way I have worked out how to achieve this is

Settings > Technical > Views

Search for "Address Details" view name

Edit the architecture directly, replacing "ZIP Code" with "Post Code" (hint - these should always be between <label> </label> tags)

Save the view. 

I'm sure next time an update gets applied that this will be overwritten.

A better way is suggested in the blue text in the window 

" Be aware that editing the architecture of a standard view is not advised, since the changes will be overwritten during future module updates.

We recommend applying modifications to standard views through inherited views or customization with Odoo Studio."

But with my limited testing I couldn't get inherited views to work. Something to work on down the track.

Hello,

Yes, you can modify the field labels and layout of the checkout page.
You can also add or remove fields based on conditions (for example, enabling B2B will make VAT fields visible). However, please note that with Odoo Online, you will have only limited functionalities.

Thanks,
info@creyox.com