Vtiger SalesOrder customizations

I need to calculate the commission for Vtiger users based on sales order with these criterias: - if the sales order is the first for the current Contacts, the vtiger user earned 50% of Total Sales order amount - if the sales order is NOT the first for the current Contacts, the vtiger user earned 10% of Total Sales order amount I have the possibility at the end of each moth to calculate (... it hink with the Reports module) the Total Commission of each vtiger User