If you are taking advantage of a "Free" trial period through a credit card or checking transaction, CCBill will request, and your bank will immediately put a "Reserved Funds" hold on your credit card for the amount of the subscription plan you have selected. For a period of up to 7 to 10 days after you cancel, these funds are considered out of your bank even though technically they are still there, and you could overdraw or exceed the limits of your account.
Please understand that CCBill has no ability to hasten this process. You are responsible for any charges imposed by your credit card issuing bank for exceeding your account limits or overdrawing your account.