You're only charge when the payment is successful. Thus, you're not charged when the credit card payment processed is declined.
There are multiple reasons for why a card may be declined, such as special characters in an address or required voice verification. The following article contains further insight on this: Why is My Customer's Card Declined?.
Additionally, I recommend calling the Merchant Services Team to assist you with determining why it was declined. Their contact information can be found at the following link: QuickBooks Payments Contact Information.
Also, you can refer to the following articles about credit card fees, as well as a quick guide to credit card processing: