grouping sub grouping basic question

This is confounding me, it's simple but I have not had this before.

I have sales data which user wants to see summary by OrderType. This means the invoice which has multilines should be summarized as well.

so we have

Inv.                      Ext$                OrdTYPE

123456                   1.00                 INT

123456                    2.00                INT

So I want my group1 to be Ordtype

and want to show

123456                       $3.00           INT

but I am getting the details how do you show this data? details line no. what group to put this in?