If you want to make absolutely certain that you won't get billed, simply cancel the billing agreement of the free trial as soon as you've started it. The free trial will remain active for the full 30 days, but when it terminates you won't be billed because the billing agreement was cancelled.
The free trial is cancelled by using My Account, Manage Access Licenses in the menu, clicking the black hamburger menu button of the Business Suite License, and then using the Cancel Billing Agreement menu option.
If you later change your mind, you can reactivate the billing agreement in the same manner.