How can I define that certain countries do not have to enter a postcode/zipcode to receive a shipping rate.
This can be set within the standard Magento settings.
If you navigate to the configuration panel through
Under the general heading there is a heading called
Countries Options. In this tab there is a option called
Postal Code is Optional for the following countries. In this option you can state which countries that zip codes need to be entered in are optional.
As you can see from this screenshot we have set Ireland to optional postcode entering at the cart: