Call their hardware support phone number, only problem i had was my package sat at their receiving house for 2 weeks before they processed replacement.
And i had the same drifting problem with left stick.
The one controller I started playing 360 with, from an Elite bundle, was the left thumbstick that was slightly off, too. My characters would move up if I didn't touch it. Now my 2nd 360 controller I can see hints that it might start this. It's a Halo Reach LE controller. I thought I was alone, as no one I mentioned this to had it happen. Good to know it's not me just pressing "up" so much that I warped controllers, lol.