LabCorp review: Debit my bank account without permission

I recently had doctor requested bloodwork completed. At the time of the blood draw, I paid my co-pay directly to the lab tech. Approximately one week later, i received an invoice from Lab Corp advising there was an additional charge and they had already debited my bank account. On checking my bank account, they had indeed taken their additional fee a few days prior. I emailed Lab Corp and requested an explanation. How can a company debit my account without my permission or awareness? Here is their response:

"Good afternoon,

Thank you for your email pertaining to Labcorp’s credit card authorization program.

When a patient's credit card information is provided at the lab on the date of service, an actual invoice for the testing is not mailed. An explanation of benefits will arrive from your insurance company. We adjust the invoice accordingly; charge your credit card for the amount that they advise is the patient's responsibility, and mail out a receipt of payment.

At this time, the credit card program is considered voluntary. If you prefer not to participate, you may advise the technician accordingly. You should not be refused service. Should you experience difficulty with this process at the Patient Service Center, please advise the technician to contact the billing office."

I never signed a credit card authorization with this company or facility. On discussing with an attorney, I have grounds for a civil suit. I post here so others can be aware.