My Scenario is similar to Micrsoft X-box example. Right now, if you go to microsoft site, you will be able to reserve a unit with a small amount of down payment. But most probably, the product is not even released status, it might be still in design stage.
But once the product is released, customers who have created the pre-orders, will get their unit first. But most probably Microsoft will send them a email with reminder to pay the complete payment first. And once that's been done, the unit will be shipped.
And after going thru all the suggested solution / ideas, I see a good match with T W.
Possible document flow -
Pre-order>Billing>Actual delivery order>Actual delivery>PGI>Billing