Grouping Under grouping(i.e. - Subgrouping)

While designing a report i am facing a problem.

I have taken a table -- shipping agent and then Sales invoice header
which is indented under shipping agent

so it looks like

shipping agent
sales invoice header

Now after linking the tables i had sorted the sales invoice header with the key combination (Bill-to City) and grouped the sales invoice header with the same key i.e. - Bill-to City

Coz i wanted to sort the record so that i can get city wise - shipping agent record, which are associated with the sales invoice and then for this particular city i want the total based on bilty no.

ex. REPORT AFTER GROUPING WITH ONLY BILL TO CITY
transporter XYZ
city ABC
bilty quantity
123 54
123 65
123 46

city DGQ
bilty quantity
549 54
549 65
126 46

I WANT MY REPORT TO LOOK LIKE

transporter XYZ
city ABC
bilty quantity
123 165

city DGQ
bilty quantity
549 119
126 46

IT MEANS I WANT A SUB GROUPING HOW TO DO IT????????