You want to use billing for a customer project.

The item category is the customizing object that controls the billing of sales order items for a customer project. The item category is a parameter that defines the basic characteristics and functions of a sales order item, such as whether it is relevant for billing or not, whether it requires a delivery or not, and whether it can be invoiced individually or collectively. The item category also determines which billing plan type can be used for a sales order item. The billing plan type is a parameter that defines how billing dates are determined and how billing amounts are calculated for a sales order item. Reference:https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html