Having checked my phone the only paid application I have at all, which had a one-off license, was the notification application for pushover.net Honestly I thin

Having checked my phone the only paid application I have at all, which had a one-off license, was the notification application for pushover.net Honestly I think I spent €3 on it, and I'd happily pay that a year to keep using the service. My automated testing-utility probes about 1000 ping/ssh/http services every 90 seconds, and sends me notificaitons when/if anything fails via that application. Pushover has been absolutely robust and reliable, and although I don't expect instant notification in all circumstances I seem to have received it.