Trouble adding courier shipping options - business is UK-based so doesn't have a ZIP code, how can I add shipping options other than flat rate/depending on weight

I want to add courier shipping options to my squarespace checkout, and the option to add them requires the business to supply a 5 digit ZIP code for item dispatch. However, my business is UK-based, and so we have post codes (different to ZIP codes). Is there a way a UK-based business can add courier options?

Hi Paul, Thank you so much for your reply - this is much appreciated!