You can easily implement, whether you customer may install an update or not. Add a new module of the type Yes/No (module type 4) with the module name Update Subscription. This module is time limited. When your customer buys your software, you set the time limitation to the number of days the updates are free - e.g. 30 days.

Your update programme just tests, whether the number of days available is > 0. If yes, the update can be installed. If your customer buys an update subscription, you just send him a key for the desired time period.

This is a very convenient way to handle updates, because you can place the update on the internet and only customers with a valid update subscription (module Update Subscription is not expired) can apply it.