I wonder what is the right configuration for a service. 

Example: We sell a number of parts that we also assemble as a service and under separate charge. 

I have tried different possibilities with service products (the product in this case would be "assembly"), but I am not satisfied with any of them. For example, a "delivery" must be created - which does not go in Deliveries but must be entered manually in the SO before invoicing. If this is forgotten, the service is missing on the invoice.
I just want a "dumb" service item that doesn't control anything and doesn't need to be controlled, but just goes through from the SO to the invoice.
Any ideas?