Your free trial does not require a credit card. It will automatically cancel at the end of your trial period unless you have subscribed to paid plan. If you would like to cancel after you have subscribed, sign in and go to the Settings > Subscription > Cancel Subscription.
If you have entered your billing data i.e. created an Envoice Billing Account or linked your company account to existing Billing account, you will be automatically switched to paid plan after 14 days or 30 free documents are used, whichever comes first.