This feature is currently in beta. 

Once you’ve enabled credit card processing, your customers already have the ability to add their credit cards on file, whenever they access their invoices online.

Now,  you can also send out a direct link to each customer via email to collect card info.

You can choose one or more customers and send the email in a batch.

To access this feature:
Sitemap -> Manage Credit Cards

Then click on “Send Card on File Link(s) “.