If the garage say you need a new drive shaft on your fortwo or roadster, ask them if it's due to corroded reluctor rings; these are not sold by smart as a separate item!
Smart only sell complete drive shafts at a cost of over £200 each!
Simply heat the new reluctor ring until it's glowing cherry red and slip it over the drive shaft. Your car will have 2 of these rings but you don't always need to replace both!
Since you are disturbing the driveshaft seal in the differential housing we would recommend that you change this too.