Order processing is divided into documents and associated statuses. Each document can have its own status and posting templates. You determine the process flow. You also define posting templates, which are executed when a status is selected.
Desired workflow from the customer's perspective:
1. we prepare a quotation.
2. the customer accepts the offer.
3. order confirmation is sent (50% of the invoice amount is due with the order confirmation).
4. a payment on account is due and sent with the order confirmation.
5. delivery is sent with invoice (remaining amount)