Show the customer a clear error message

JavaScript example

const REASON_MESSAGES = {
  OUT_OF_ZONE:       "We don't deliver to this postcode. Please check the postcode or choose a collection option.",
  UNKNOWN_POSTCODE:  "We couldn't recognise this postcode. Please double-check and try again.",
  MIN_BASKET:        "The minimum order for this delivery area is not met. Add more items to qualify.",
};

function getErrorMessage(reason) {
  return REASON_MESSAGES[reason] || "We can't deliver to this address. Please contact support.";
}

// usage
const data = await checkZone(postcode, basket);
if (!data.canDeliver) {
  showError(getErrorMessage(data.reason));
}