I am fuming with Apple, how do I make a formal complaint?

A month ago I cracked my screen of my iPod 4th gen, now, a lot of my friends have cracked screens too because it's so delicate. The difference is, mine was in a case, and I've had it less than 3 months, it dropped less than a meter. Everything still worked including the front and back cameras, up until a week ago where I've been experiencing small glitches, such as the unlock button freezing, the slide to unlock becoming unresponsive, and my wifi choosing when to connect fully and not work, I know my router has nothing to do with it because it works perfectly everyday on my laptop. I went to the genius bar today to ask if they could fix these glitches, and they've said they can't because my screen crack voids the warranty, they said they can only replace, not repair, and for that, it would cost me �126. I am NOT paying that much, it's catch 22 with Apple, because if I went and got my screen repaired elsewhere (because apple don't offer it ANYWAY) they would say that the original state of it has been tampered with, and they would refuse to fix the glitches. How do I make a formal complaint?