Page Comparison

A task needs to be enabled in order for the customer to get Invoiced.
To activate the task go to Administration> Advanced> Tasks> Invoice Pre Ordered Products turn the check box on 'Enabled' and click on Save.
Enter the appropriate value in the Schedule field and to do so see link Setting task schedules.
The task will run based upon the values set on the count cycle selected.

Quantity Invoiced formula for Quantity Invoiced = Unit Quantity\Number of Invoices
Value Invoiced formula = Unit Quantity x Unit price ex tax \ Number of Invoices.

The billing rate is separate to the usage rate and the usage rate has to be the same values as the line that is getting credited.

A product needs to be set up to be used in the credit lines generated by the preordered contract product. See more details below.

In Preordered products on the Used Shipment Lines tab, displays a list of the Usage Shipments Lines and Redemptions when the service is used up.
In Preordered products on the Invoiced shipment lines tab, displays a list of shipments when the customer has been charged for the pre-ordered product.

Need to set up a product to stand for the Preordered Product Redemption and maybe call it - Preordered Product Credits. 
You can only have 1 product set up with a type = Preordered

The credit redemption product needs to be set up as a type = PreorderProduct
The product that is preordered can be any product

Set up a product to be used in the redemption credit lines generated by the pre ordered contract call it PreOrderProduct with maybe a description = Preordered Product Credit
Ensure the product has a Type = Pre Order Charge
Assign it to the Non Stock Product Category - Pre Ordered Product Redeemed
You can only have 1 product set up in the system with a type = preordered charge