Orders with Deposits and Prepayments

I am new to NAV and I am trying to automate a prepayment process.

We build machinery and we billing the customers in 3 steps. First, 50% deposit upon agreement of contact, 40% when the machine ships and remaining balance 45 days after shipping.

I tried using the Prepayment option in sales order, but it only allows one prepayment. Any other suggestions on how to do this so its a somewhat clean and easy to follow process.

Ideally I want to create the SO so I can generate the WO and get a deposit. Close the WO, ship and bill for the 40% due when the goods leave and then bill for the remaining 10%.

Any ideas or suggestions or comments are welcomed!