Billing of Services 
This section provides information about the specifics of SAP CRM billing for service transactions.
For sales items entered in service transactions, a sales order is created in SAP ERP so that the delivery can also be processed in SAP ERP. In order that these items are billed in SAP CRM, you need to ensure that they are not flagged as billing relevant in SAP ERP.
Note
For sales items in service transactions, no sales order is created in SAP CRM .
Service contracts that are valid for service order items
If a service order item is covered by a service contract, it is billed at a flat rate using a billing plan, according to the contractual agreement.
Service confirmations created as follow-up transactions to service orders
Fixed price agreement
If you have agreed on a fixed price with the customer, you need to flag the service order item as billing relevant, but not the service confirmation item.
Resource-related billing
If you want billing to be resource-related, you need to flag the service confirmation item as billing relevant, but not the service order item.
Note
If you want to bill a service confirmation that was not created as a follow-up transaction to a service order, you need to flag the service confirmation item as billing relevant.
The Billing Relevant flag is not visible by default in the CRM WebClient UI, however, you can configure the interface to display this flag.
Data is transferred to the billing due list when a transaction is saved, with the exception of items with the billing relevancy Transaction-Related Billing After Completion. These items must be explicitly released for billing in the CRM WebClient UI on the Release for Billing page.
The following figure shows the procedures used to bill service transactions:

Billing (documentation for generic billing functions)