Your home page has the price for the Extension Plug-in for $39 and says that buyers get updates for one year. What happens after that year? Is there an annual fee to continue to receive updates? How would I continue to get updates?
if you are thinking that plugin will be locked after that period it will not be the case. After one/two years it will still be working unless there are some big changes for example Google updates the way of syncing Calendar events or Twilio change the API access. So for that case you will need the latest update. Regarding a way of keeping your Extension up-to-date that is possible via regular update on plugins page inside admin panel of your WordPress site. So as soon as new version is released you can simply click on update the plugin.