In your Selling Channel settings, there's a dedicated field that will add a message to the bottom of packing slips automatically. This field can have different values for different stores. These steps below will help you set the this value for one of your stores!
Head to your Account Settings (the wrench icon in the upper right).
Next, select Selling Channels and then Store Setup from the sidebar on the left.
Now you will see the list of your connected stores. Beside each store you will see an Edit link.
Click Edit for a store that requires a packing slip message, and then under the "Packing Slips" section you will see this field.
Add the desired message (up to 2000 characters) and Save Changes.
That’s it! Repeat the process for any other store that requires a packing slip message.