After you create your first invoice, InvoiceBot will ask you to choose one of the plans above. Pick the monthly plan and you'll automatically be signed up for the free 7-day trial. After the trial the Apple will automatically start to charge you.
In case you're unhappy with InvoiceBot, you can go to Settings > Your Account > Manage Subscription. This will forward you to Apple's subscription page, choose InvoiceBot, scroll to the bottom, you'll find a button to cancel your subscription.