Hey Ben,
Circling back here after finding a similar reddit thread talking about this, like I mentioned in a previous comment our app does exactly this! We let you set up rules to add a certain to product to every order, only orders with a subtotal > a specific amount, and only after a customer has spent a total amount or placed a certain number of orders!
Link to app: https://apps.shopify.com/pls-smart-pack-ship
Here’s a short video demoing me setting this up based on the reddit thread:https://youtu.be/ZPz9aIby0mg
Thanks,
Tom