Hi Luke,
Thanks for posting this out on the community; it’s a good question! At the moment, there is no way to add an additional cost to the expected packaging cost for your products, but we can certainly see how that would be helpful. There are a couple of Feature Requests open which are similar to what you are looking to do. I would suggest taking a look at the feature request Expected cost and non-stock ingredients/utility costs, which I think would solve your problem and would be worth adding your vote and comments about 3rd party packaging to, and Auto-allocate packaging costs which you might also be interested in.