I think if you send an email to "us_dhs_notify@dell.com", they will forward the request to the propper department. I got an email from them telling me an order of my was being delayed, and they gave me that email if I had any questions. I responded saying I wanted to cancel the part of the order that didn't ship. I checked my online status 2 days later and it was cancelled.
You can probably call them at 1-800-624-9897 as well...