If your client has paid with their Credit Card you can choose to save their card on their profile with your company.

From the "Billing" Tab you can select the three dots next to their 'Online' Credit Card payment and select 'Save to Contact'.

Now you'll be able to capture a charge to their saved card on all their future projects.

You can also see any Saved Cards when viewing your client's information under their 'Billing' section.