SUM Sales by Month and Customer

Hi All

I just joined the forum to try to find a solution to a problem I've had for years.

What i'm trying to do is find out how much I do in sales per month for each of my customers. I currently use this formula to give me my sales per month =ArrayFormula(SUMIF(MONTH(SHIPPED!K:K);1;'SHIPPED'!I:I)) but I would like to be able to break it down by customer.

CUST DATE SALE
100 12/1/2012 $125.00
100 12/20/2012 $50.00
100 1/5/2013 $25.00
200 1/6/2013 $200.00
300 1/7/2013 $150.00
300 1/9/2013 $75.00

so for the above date the formula would return for customer 100 in month "12" $175 or customer 300 in month "1" would be $225

Is this possible ?

Thanks
George

Not yet I was hoping to do it with a formula so I can use it in other sheets. Also I forgot to mention I'm using Google Docs spreadsheet.