If you want to include storage units in your monthly billing plans, here is the way to do so that also allows you to keep track of storage inventory.
Go to your Settings tab, then click on Storage. Make sure you have the all relevant storage types and all the possible units accounted for. If you need to add more, here is more info.
Now go to your Clients tab. Click on "Storage" next to the Client's name, for the Clients that are on plans.
If you want to offer a shelf of storage, for instance, add a unit to each of their accounts so that the system is tracking the units of storage that are applied to Clients. To do so, click on "New Storage Unit Rental" and enter the number of units and choose type of storage you wish to apply.
Next, go back to your Clients tab. Click on "Fee" next to the Client name, then "New Fee/Credit."
Here, add a monthly credit to each Client's account that will offset the cost of the unit of storage, since they will be receiving this as part of their plan. Describe the credit and indicate the same amount as what the unit typically costs. Choose "credit" and "recurs monthly."
Click Save. Repeat this with all your Clients on monthly plans.