I am grouping by customer and then by invoice

GH1 Invoice | Amount
D         1 | $10
GF2     1 | $10
D         2 | $5
D         2 | $5
GF2      2 | $5
GF1   Total | $20

I want to sum the Amount in the GF2 field for the whole group.  However when I do I get $20, when I should get $15.  How can I make this sum work?

I tried creating a new formula to create data that would take into account repeat instances:
if
  {#count of items on invoice} > 1
then
  {@amount sum} / {#count of items on invoice}
else
  {@amount sum}
 
However when I tried to sum this field I got an error message that the field could not be summarized. 

I know there is a distinct count, but there is no distinct sum.  I want to sum the values for GF2 not for all of the detail rows. 

Thank you so much!