Plugin subscriptions are complete separate from your main subscription to Mavenseed, and are offered through the Maven Market on select plugins. Each of Premium plugins will have their own price, and are charged as separate subscriptions based on the number of sites that you would like to activate the plugin on.
You can have multiple plugin subscriptions each with their own recurring payment date. These can be managed from the "Addons" tab within your account.
For each plugin subscription, you can update the plan or cancel it completely. Your plugin subscription will remain active through the end of the plugin subscription billing period.