Thank you so much for being a part of our community, we are so happy to have you here with us.
I am thinking that the best option for this might be to use a label message since it is supported for UPS. This can be configured in the document options, and you can assign a field replacement like Notes from Buyer (corresponds to Customer Notes) to display notes like this.
If you want to learn more about this I would check out the following article.
Please let us know if you have any questions or concerns.
-Erin: Your Friendly Neighborhood ShipStation Community Manager.