Adding to summary totals from different objects

Ok, this is kind of complicated to explain. I have two objects setup for my clients. One allows them to register policies which we charge an admin fee for and the other object allows them to order POS for their business. They are billed monthly for both according to the policies they have sold and for any POS ordered.

The problem is when I try to create a page as a printer friendly invoice a can't add the two 'summary totals' together. 

Does anyone have any custom code that could add these two total (summaries) so that my client sees Total DUE? I've set everything up but just can't get these two results to play with each other.

Many thanks in advance.