Hyde here from Shopify. Interesting question!
You can edit the style of the checkout but also the content/forms to some extent as outlined here.
The essence of the form editing you can do is here:
Your customers should be able to include their Comuna in the second address line. If needed, you could always stipulate that it is a requirement for shipping purposes by editing the language using the language editor. You could edit the language so that "Comuna" is included as a prompt for your customers in the filed:
I hope this helps!
All the best, Hyde.