Iggi No reason at all why you shouldn't return it. I do. I just save all my free return labels so I've got plenty (easy now they send 2 with each purchase) and if the price drops just send it all back and re buy it.
Once I rang them up about further discounts on stuff I had bought and they told me to do this, I did think it was a bit of a waste of their postage costs though. For reason for sending back I put 'Went down in sale' 
This is perhaps why I didn't get a code.