Please reference the Terms of Service you agreed to when you signed up:
"Your HelloSign/HelloFax Premium Account will continue in effect unless and until you cancel your Premium Account or we terminate it. You must cancel your Premium Account before it renews each month or year in order to avoid billing of the next month's or year's fees to your credit card."
As long as you cancel your subscription before your renewal date, you will not be charged. Otherwise, you will retain access to the premium features through the end of your paid period.