Let's say you want to include an optional fee for setup and teardown of your chairs in your inventory.

The first thing you should do is create a Chair setup fee and Chair teardown fee as a Service in Inventory. Learn more here.

You can add those as associated services to your chairs, and you'll be prompted to add them in the appropriate quantity on contracts.   

As you add the inventory item to your projects, you can view associated services by hovering over the item name and clicking on the grey box that appears.