Hi @lorenzoc ,
Hyde here from Shopify. Interesting question!
You could obviously give a very brief explanation within the shipping title itself when creating the shipping rate. In other words, when it comes to point for in the steps to create the rate as outlined here, you could enter an explanatory name:
For example, the name could be “free shipping for such and such a reason”. The alternative would be to use the language editor and edit some of the wording in the checkout. Simply type in “shipping” into the search bar and you’ll see all the different options that you could edit to clarify your shipping
I hope this all helps! Let me know if you have any further questions.
All the best, Hyde.