Invoice add-on can be used to create invoices within projects and collect payment from clients. WP Project Manager lets you assign client roles to users. So when you have finished your project, you can create an invoice for your client. The invoice will be automatically formatted based on your input and will be sent to your client when created.
Clients can pay against your invoice. The amount charged will be added to your account and the invoice will be automatically marked as paid.
You can check out some screenshots to see Invoice add-on for WP Project Manager in action.