How to create a postal code selector.

[danielPetcov]

Hi guys,

I want the user be able to choose, on cart page, only from some predefined postal codes where his delivery to be send. The selected postal code should be somehow reflected in the checkout page. Either fill the postal code field and block it, or give an error because the postal code values are not the same.
I would like how can I implement something like this bymyself, without the need of using an app.

I know that without a Shopify Plus plan i can not edit the checkout page, but is there a go around this thing.

Thank you.