I am not sure what the problem is and what you are trying to achieve.
Let me try and clarify:
In Clarizen, you can have fixed price\ fixed cost items as well as variable, based on resource rates.
Both are summed up to the relevant fields (expected cost\actual cost, expected revenue\actual revenue) from any level to the top most level.
Fixed price\Fixed cost items are automatically summed up to the actual revenue\actual cost only upon completion, while the items that are rate based are summed up as soon as the actual revenue\actual cost is updated at the level of any work item.
Are you looking to sum up only fixed price items? Are you looking to aggregate to upper levels invoiced amount?
There are plenty of ways to handle any of these scenarios. Some of it are supplied out of the box. Some can be done through fairly easy customization.
(Sorry for the long reply)