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.

Number of Months = Total Unit Quantity /Number of invoices. 
You must use an approximation. Unit Quantity must be a multiple , this is how the arithmetic works.

If you are converting over to Readysell preordered products with the following opening position:
Last Invoiced Oct 2017
Contract Ends July 2018
That is 9 months of Invoicing remaining
This customer currently has 14.5 hour remaining
Invoiced 2 hours per month
9 months <> 14.5/2 as 14.5/2 = 7.25 months
You must use an approximation that is Unit qty = 14 and number of invoices = 7 to get charged 2 hours a month.

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.

Quantity Available = Balance remaining on this preordered product that has not been consumed by a posted transaction such as a sale shipment or service shipment

Quantity uninvoiced = balance of this preordered that has not yet been invoiced to the customer. Calculated as the unit quantity of the preordered product less the quantity that has already been invoiced.

A product needs to be set up to be used in the credit lines generated by the preordered contract product (Preordered Product Redemption) and maybe call it - Preordered Product Credits. 
You can only have 1 product set up with a type = Preordered
See more details below.

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