It appears that sometimes when you begin a subscription, even if the subscription has a free trial, that this can sometimes trigger Apple to collect any deferred payments you may have made when purchasing other Apps or subscriptions or items.
So while the Strongbox free trial doesn't cost anything you can sometimes get a message that Apple has taken a payment at the time you start the free trial. This will never be for the Strongbox free trial it will always be for something else. It is just Apple deciding to process payments at that time. This can be confusing and distressing but we're afraid there is nothing that can be done from our side here.
You can view the transactions on your Apple account and see what it was you were charged for.