This is Dallas. I am one of the members of the Social Care Team here at Shopify.
At the moment there isn't a way to edit the code of the checkout page in Shopify. The best option, in this case, would be to create a popup box before your customers reaches the checkout page.
The Products Warning app will be able to help with your pop-ups. However, it will just show up when your customers add certain products to the cart.
I am not entirely sure what you are needing/wanting with these pop-ups so another one to look into is Easy Box. This app allows for link-based pop-ups.
This app is more of a promotion based pop up, hence the name Promotion Pop-up, but it allows for cart based popups.
Easy Popup is an app with a lot of features, so it may be worth looking into as well.
Since I am not sure what exactly you are hoping to share with your customers with the use of this pop I am not able to direct my suggestions in the most specific way. If you are able to clarify a little bit more what your goals are, I would be happy to see what else I can find for you.
I hope this helps with what you are looking for! If you have any other questions or concerns please don't hesitate to let us know. Our support is here 24/7.
Thank you for replying to this post. I will definitely check out those apps for future references. However what I'm trying to do is when a customer chooses UPSP's First Class Package as their delivery method, I want to warn them that this method is "blah blah blah would you still like to choose this method?"
If this is not possible, then I want to know if there are any other ways to effectively warn the customers. For example, have a small note right next to where you choose the delivery method so that customers would be able to easily notice it.
It isn't possible to add any note when a shipping option is chosen. The cart.liquid file isn't able to be adjusted at all.
For what you are wanting I recommend doing a product or cart pop up. This will allow you to make the not for your customers to read in either location. The apps that I gave above are great for this.
If you would like more help, let me know!