Trying to Calculate the difference between 2 columns in pivot table

Hello everyone!!

So I am running in to a large brick wall! I have a single pivot tablet with a column for items received and then a column for items shipped. I would like to create a calculated field in the pivot table that will subtract the items received from the items shipped columns. I have attempted to add the calculated field using the tool bar but it keeps giving me "0" for every item on the pivot table when there are obvious differences.

Does anyone have any ideas or a solution for this????

I have added a sample workbook with the data I am using to do this.