Is the monthly fee of $9.99 the only charge for the use of the service? Are there any device purchases or rentals?

The $9.99/mo covers everything, regardless of which device you prefer to use when summoning service (the button, mobile app, web application and personal online dashboard, or telephone—all of these devices, apps,  equipment, websites, etc., are all part of the service. It is a month-to-month deal. No contract or long-term obligation. Paying members are charged $9.99/mo, billed quarterly.