You guys shouldn't advertise animated heads then.
It's really not fair to charge 20,000 gralats and state that animated heads are allowed (unless you provide a full refund). Even if you just allow them to choose another head, it's not really fair because they may have only paid the high price to have the animated head. Another kind of head isn't really what they paid for, in their mind.
I'm not saying I disagree with Stefan's reasons though... I'm just saying that you guys can't have your cake and eat it, too, so you shouldn't try to advertise it like that.