Pivot Count Unique values - Excel 2010

Hi,
I am trying to analyse a data set by summing the values per quarter but also the amount of invoices making up that sum. The Count Option on the pivot simply counts all entries which I do not want.

I got a solution for the issue (=1/COUNTIF($A$2:$A$31,A2) then PIVOT the SUM, but it wont work on my original data as there are over 20,000 lines. The Pivot returns a sum of #DIV/!

The attached has sample data including the solution that works well on this size data.

Hope someone can help....thanks.

Thanks a lot, thats awesome.