using sumproduct on subtotals meeting certain conditions

[Jeff Manning]

How to sum all the negative subtotals on a detailed data list:

Col A Col B Col C
customerA inv1 100
customerA inv2 100
customerA inv3 -100
customerB inv1 100
customerB inv2 -200
customerC inv1 -100

If I subtotal by customer, I get:
customerA 100
customerB -100
customerC -100

I would like to summarize only the negative subtotals (result -200)

What would the formula be to accomplish this?