The ability to pay for a service using existing credits was added in v3.3.0. As an admin, you can go to the client's profile page and click Record Payment under Actions, where you will see the option to Apply Credits to invoices (in the same currency as the credit).
As a client, when placing an order, there is a similar checkbox shown on the page where they select a payment method if they have any credits available. And the client can then pay for the service using the credit from their account.