Pay for what you use.
No, you don't need a card to sign up. You will be use the service and all of the features during the trial period. After the trial period is over, you will need to put a credit card on file to continue using the service.
Yes, you can try out the service and all of the features free for 15 days.
No, you will have access to all the features of the service during the trial.
Yes, you can cancel at anytime.