How do you know what goes into each kit? The solution above by Luis will give you what you need with what's presented, but it comes with a concern: Your association is completely random. It's going to sort out in whatever order the database coughs it up in, and won't deal with, for example, two kit sets repeated 3 times each. At least not well.

Your example is oversimplified for a production system, because it just can't work that way long term because there's no associations outside of a single table's row order. If you've got other controls already making sure that you're restricting it to a single kit type for a single invoice (I'm making a crapload of assumptions here), then Luis' solution will work. Otherwise, you'll need to get us a more holistic view of your schema to help you not shoot yourself in the foot.