The customer is always right.

In their own mind yes they are. So, feel free to take advantage of these people. Pissing them off further will probably lead to a boycott of your store/company by them alone, and put their bad biusness at your competitiors. Also, when taking advantage of this, ensure they waste as much money as possible. "Sure sir, that extra $2500 card is needed in your server to control the hard drives, even though there is an included solution that you won't pay attention to".