I'm not really sure if they can.
They sent me an order acknowledgement.
They thanked me for my order.
They sent me the terms and conditions of sale which include;
This Agreement contains the terms and conditions that apply to purchases by Home, Home Office, and Small Business customers from the Dell entity named on the invoice ("Dell") that will be provided to you ("Customer") on orders for computer systems and/or related products and/or services and support sold by Dell in the United States. By accepting delivery of the computer systems, other products and/or services and support described on that invoice, Customer agrees to be bound by and accepts these terms and conditions. If for any reason you are not satisfied with your Dell-branded hardware system, you may return the system under the terms and conditions of Dell's "Total Satisfaction Return Policy", which is located online at www1.us.dell.com/content/topics/global.aspx/policy/en/policy?c=us&l=en&s=gen&~section=010 or may be found in the documentation accompanying the system.
THESE TERMS AND CONDITIONS APPLY (I) UNLESS THE CUSTOMER HAS SIGNED A SEPARATE PURCHASE AGREEMENT WITH DELL, IN WHICH CASE THE SEPARATE AGREEMENT SHALL GOVERN; OR (II) UNLESS OTHER DELL STANDARD TERMS APPLY TO THE TRANSACTION.
These terms and conditions are subject to change without prior written notice at any time, in Dell's sole discretion.
1. Other Documents. Other than as specifically provided in any separate formal purchase agreement between Customer and Dell, these terms and conditions may NOT be altered, supplemented, or amended by the use of any other document(s). Any attempt to alter, supplement or amend this document or to enter an order for product(s) which is subject to additional or altered terms and conditions will be null and void, unless otherwise agreed to in a written agreement signed by both Customer and Dell.
I don't care that the order acknowledgement included an additonal clause. Their terms and conditions preclude it.