If you want to automate this, you can use Order Automator app. We have people doing exactly that - adding to the Notes based on the order details, so it goes on the packing slip. It's customizable, feel free to check it out and reach out to support at orderautomator.com/contact to get dialed in. There are also a handful of other features to help automate tasks.
Another thing you could try is Shopify Flow. I'm not too familiar with it but pretty sure you can identify orders based on source and edit the notes.
