I agree that you should just let it go. Submitting supplementary materials, especially ones "explaining low grades," is likely not to be taked into consideration or at all, or viewed very positively. If the rest of your application is exceptional enough, you'll be admitted despite your grades and even with the explanation, they probably wouldn't have cared about it unless the rest of your app stood out.
As previous poster said, perhaps bring it up in an interview, but otherwise let it go and wait it out. Believe me, I understand how agonizing it is.