Can I activate the Stock Transfers and Inventory Tracking plugin again?

Can I activate the Stock Transfers and Inventory Tracking plugin again?

Yes, you can always choose to reactivate the Stock Transfers and Inventory Tracking plugin. However, there are 2 possible scenarios here.
Scenario 1: Your reactivation date is BEFORE the end of your current billing cycle. In this case, you just need to click the activate button on the Stock Transfers and Inventory Tracking plugin tile (if you are coming from the Plugin screen) OR put a tick on the activation checkbox on the plugin tile (if you are coming from the Manage Account section) to activate it.
Scenario 2: Your reactivation date is AFTER the end of your current billing cycle. In this case you will have to again subscribe to the Stock Transfers and Inventory Tracking plugin through the normal subscription process. 
Plugins in Tagrain are either Subscription Based or License Based. So the price that you pay for your plugins is based on the number of POS terminals that you have in your setup and the number of licences that you have acquired. 
There will be no change in your subscription amount in the current billing cycle if you are only deactivating the Stock Transfers and Inventory Tracking plugin AND not updating your Store / POS count.