You're not! 

When you first subscribe to our Pro service on either the annual or monthly plan, your 7-day free trial starts immediately. When this happens you'll get a receipt from Apple stating that you've started your trial, but you're not being billed yet.

Once your trial is over, and if you didn't cancel it before it ended, your paid subscription will begin automatically and you'll receive a second App Store receipt. This one will reflect the amount you've been charged for the year or month, depending on the type of subscription you selected when you started your trial.