I only want to offer free shipping IF they also purchase an in-stock itemUpdated 2 days ago

With our Pre-order Override Rate, you can offer Free Shipping for pre-orders IF they are also purchased with in-stock items. And if they're purchased alone, they'll be charged the standard shipping rate you've set up in your dashboard.

Head over to the Settings tab on the left-hand side of your dashboard and click Shipping. 

Click the Add override rate button and it will bring up a window where you can name the rate, add the price and select the required countries. You can also add price conditions (eg. free pre-order shipping if over $50)

Click Add item type condition, tick Requires buying an in-stock item and hit Save.