I want to offer hourly packages, so it would be nice if I could create an advance invoice in (Billings) Checkoutpage, which is then invoiced in Books. 

Important, I don't want to do this in the backend itself. The customer buys via the product checkout page.

For example, a customer buys 10 hours in advance and receives an invoice. A project is created and the customer has a credit of 10 hours. Now hours are recorded in the project, at the end the customer receives an invoice if he has used more than 10 hours.

Is this possible in Books / Billings?