is everyone missing the fact that he bought it 1 and a half months ago and he bought it with 3 months warrenty !!!
The gararge should buy the new box and fit it for free !!!
Its very good of you to buy the gearbox yourself and if i was the gararge, i would fit it and send you on your way thankful i hadn't had to buy the box.
If he pisses you about sue the cunt !!! you will have a strong case as long as you can prove you didn't hit the curb.