The free trial is absolutely and completely free. It is done via a $0 (zero dollar) "purchase" on your Apple ID inline with Apple's guidelines (we wish there were a better way to do this). There is no subscription or obligation to continue after your free trial.
It appears that sometimes this $0 transaction 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 should be able to view the transactions on your account and see what it was you were charged for.