This is an accepted solution.
Ash here from the Shopify team. Welcome to the Shopify Community!
I understand you want to add a notice for customers to let them know that you don't ship to PO boxes. This is absolutely possible to achieve, and I'll provide some steps below for you to follow. In order to add a note to this area of the checkout, we're going to be using the Language Editor to add/change some wording in your theme. Here are the steps:
Another place you can leave a notice is above the shipping options available in the checkout. This may be handy as a second notice in case a customer still enters a PO box and proceeds through the customer information step of the checkout. To add a note above the shipping options, search the Language Editor for "Shipping method notice". Here's what that looks like in the Language Editor:
and on the live checkout:
Since each theme is different, there may be slight variation in the exact names of the fields you need to edit in the Language Editor. If you have any trouble locating the areas you'd like to leave a shipping notice, please let me know by replying here and we can take a closer look together.