Hi All,
I have a free solution to this. Using Cognito Forms (free plan available) you can embed a form into your squarespace site. When they place a postcode in, the form drops down custom text.
There is a little background work required with what you want the text to say when the post code is submitted but it is relatively simple and customised.
See example here, try post code 6000. Then try postcode 4000 for example. https://www.yellowlinecoffee.com.au/shipping
Let me know if you need any more info if setting up.