How to fetch Invoices booked but not paid yet

I have made an app which have 4 steps:

1. Get orders

2. Upgrade selected orders to invoices

3. Book selected invoices

4. Create debitorpayment for the invoice

But in step 4 I cant see how to get invoices which are candidates for the step, so I use this:

IInvoice[] invoices = _session.Invoice.GetAll();

But then I recieve too many invoices and the may be inserted in CashBook multiple times.

How do I do this correct ?