You don't actually "subscribe" to PicSafe PRO, you purchase access for a set time period. There is no auto-renew so you don't need to worry about forgetting to cancel your subscription!
Two weeks before a subscription expires, the app will show a message asking you to renew. If you don't renew then, it will send a second reminder three days before it expires and then one more time after it expires. If you want to "unsubscribe" from PicSafe PRO simply take no action when you see these reminders. Your access to PicSafe PRO will automatically be removed once the current access period ends.