You can easily create a billing item upon task completion. 

1. To begin, go to the Tasks module.
2. Select the Task you wish to work with. Note that this does not work with multiple tasks.
3. Update the Task Status. 
4. Select the Create billing item checkbox.
5. Click the Set Status button. 

You will then be taken to the New Billing Item page where you can update any of the information prior to saving the billing item. For full instructions on entering billing items, click here.