Totaling multiple criteria on different worksheets

Hello all, I am not sure if I am even using the correct formula or if the there is a better one suited for my needs. I have a report that has a Customer column, Product column and an inventory quantity column. Here is an example of the formula I am using to collect data from another sheet in the workbook:

=COUNTIFS('Ordered Product'!$A$2:$A$249,"Customer Name",'Ordered Product'!$F$2:$F$249,"Product")

The formula accurately counts the amount of times the product shows up with the customer name. The problem is that some of the times when the product shows up in the list, its actually a negative. So for example, it shows up twice in the report, once as a 1 and once as a -1. The formula counts 2 instead of 0. How can I sum the number within the formula to accurately count the inventory?