just call us up or send an email etc. If you call us up the adviser will ask you to send notice of your cancellation in a durable medium such as email etc.
You are making assumptions without the whole story.
At the time of order the customer paid £799 for his gtx 690. We have these cards on order and this is the price he will pay if he waits for this card. The GTX 680 cards have always roughly been about £420 - £430. The price of the GTX 690 has gone up subsequently because of various factors, dollar rate, supply / demand etc. However, the only factor here is that the customer paid £799 for his card, and will get a gtx 690 for this price if he is willing to wait for it. If he wants to change to 2 x GTX 680's then he gets £799 to put towards them.
The simple way to test this, and ensure there is no deception, is to simply re-spec the pc he has ordered with 2 x gtx 680's and check the price. He will find it is more expensive.