You can create multiple rate plans to charge a monthly fee as a recurring fee, and an additional storage fee as a usage-based fee.
When setting up your product catalog, you can specify the type of product you are offering along with the rate plans of the product. The rate plans contain the type of charge for your product. In this case, the monthly fee will be setup as a recurring fee and the storage can be setup as a usage-based fee. See Product Catalog Concepts for more information about product rate plans.