Not sure about happens with the Ethernet adapter - but you'll notice that when you added the Back To School card to your basket you didn't get it free, it charged you for it and deducted some money from the cost of the MacBook Pro to bring the total price paid up to the cost of the MacBook Pro.
Therefore, if you cancel the MacBook Pro, you'll get the amount you paid for that as indicated on your bill (i.e. the cost of the MBP - the cost of the gift card).
Ah, nice explanation, thanks. I hadn't even noticed that's how it worked. So in other words I can't cancel unless I somehow return the gift card. I guess I'll just be patient and wait for my online order to arrive. I've had this computer I'm typing on for 5 years, I suppose I can wait a couple of extra days, even a week if that's the case, before retiring it.