An item number can be created and designated as a "Service Item" type. An item of the type does not have a Stocking record or a Virtual record. The use for such an item would be invoice a customer for an internal service such as labor fee, handling fee, or an extra fee you desire to pass on from a vendor.
By their nature, a service item cannot be backordered. When a service item is input on a sales order line, you can set Quantity Ordered to any value and the Committed column will fully commit that value. You can also edit the committed value before processing the order for shipping and invoicing. The Sales Invoice will be generated for the quantity which had been committed.
The sale order for this line is no longer accessible for additional invoicing, even if you had set the Committed quantity to be less than the Ordered quantity. A service item line cannot maintain backorder ... the line is simply inactive at this point forward.
SUMMARY: When you place a service item on a sales order line, it will be invoiced for the committed value on the 1st invoicing event for that line. It cannot subsequently be invoiced or edited.
However, you can enter multiple service item lines on a sales order and control when they get invoiced. Let's assume you have an item for which you will be shipping under two shipments. 20 Ea will be shipping now, and 20 Ea will be shipping at a later date. For each shipment you wish to invoice a service fee of $5.
input the item for 40 Ea and commit 20 Ea. Input two service item lines on the sales order for Qty 1 Ea and Price $5. Both lines will initially display 1 Ea as committed. For the second of those service lines, set the PickOnDate to a date beyond 30 days. You will receive a prompt asking if you would like to make this a Future Line. Reply "Yes", after which the committed value reverts to "0".
The first invoicing event will invoice the item for the 20 Ea committed plus the first service line of $5. At a later date when committing the item line for the remaining 20 Ea, edit the PickOnDate for the second service item line to be the current date. This changes the line from being a "Future" and automatically commits the 1 ea. This second invoicing event will invoice 20 ea of the item plus the the other $5 service fee.