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 either by clicking Create New when creating your chairs:

 Or, under Services in your inventory. 

You can add those as associated services to your chairs so you'll be prompted to add them at 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.