At the beginning of the subscription period, you are given the service for free. Once your free trial ends, you will be automatically charged the annual fee, unless you cancel the subscription manually prior to the end of your trial period.
You can do it by using the special link provided in the FastSpring order confirmation email.
Quantities and/or features of the underlying plan can be changed by buying a new subscription. The underlying subscription should be canceled to avoid double billing.
FlexiHub is licensed according to concurrently connected devices. For example, you can install it on 100 machines but pay for a 10-connection subscription if you anticipate only 10 devices being connected at the same time.