Pre-paid monthly rate:
When you place an order for service you will be given the option of a recurring payment frequency. You may choose to pay month-to-month, quarterly, bi-annual, annual, or every 2 years. The more pre-paid in advance determines the amount of discount that will be applied. Discount rates are clearly displayed during the order process.
Disk space storage:

The total disk space storage, listed in megabytes, for all HTML and related web site files, emails, log files, and stats. Site owners requiring more space can simply add-on storage as needed for $5.00 per 50 MB.

The WebControl panel also permits a site owner to optionally set a "quota limitation" for each Email, and FTP user which helps prevent the storage space from being fully consumed without the site owner's knowledge.