Multiple Customer Billing

 All,

Following is the scenario.

We have Projects which deal with multiple customers. In this case, when the billing happens, we want to override the % allocation which is the standard feature and bill a specific customer per Milestone / Event. 

Is there any standard way that Projects module offers to deal the situation or is Extension the only way to handle this.

All your jhelp in this regard is highly appreciated.

Cheers,

Kalyan