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 to their 'Online' Credit Card payment and select 'Save to Contact'. **Make sure you have your client's approval before saving!**
Now you'll be able to push a charge to their save card on all their future projects.
You can also see any Saved Cards when viewing your client's information under their 'Billing' section.