Bundles.app ‑ Inventory Sync

This did exactly what I needed and exactly what they said in the description.

If you already have a product set up that comprises a number of other products, this app will handle inventory management for you. Every time you sell the bundle product it will update the inventory of the products it contains. It is super simple and super easy - I installed it and had two bundles up and running in less than 5 minutes. It couldn't be easier and they've made the interface really good.

Highly recommended