Sellix does not have an event for when a subscription expires. The closest to that is subscription:cancelled, however the user can manually cancel the subscription themselves, triggering the event. The only way around this is for the developer to implement their own method of cancelling the subscription by checking whether the time for each of their users subscription has run out, excluding the users who have paid for the next month. I think a subscription:expired event should be added and called when the subscriptions time runs out and a buyer has not paid for it.