NSAW using calculation/key metric formulas in pivot table

Hi,

I am trying to create an easy way to see the average item sales rate. I want to take the total dollars sold / total quantity sold so I can get this average rate. I created a calculation with the following formula.

SUM(Item rate * shipped quantity) / SUM(shipped quantity)

When I add this into the pivot table, I assume it is summing the value because it is a very large number. I wanted to try average but there is no option to change the aggregate. I wasn't sure the difference between a calculation and a key metric so I create a key metric version called Key Metric Average Rate and I now get the menu option but all the options are grayed out and not selectable. Any idea how I can get my pivot table to show the SKU and its average sale rate? Instead