Hi @auora ,
This can definitely be done. Typically you can utilize Shopify’s Language Editor and add or alter a pre-existing message. If you are able to describe where you want your message and what the message is then I can definitely guide you further.
For example, if you navigate to Settings (bottom left corner) > Checkout > Checkout language / Manage checkout language. You will see a search field where you can enter “shipping method notice.”
You can add a message into this field as shown below:
Which will show up in this space here:
Another option would be the use of an app such as Pop-Up Window to add a message that can’t be missed!
Hope that helps. Let me know if I can clarify any of the above for you.

