If I'd known they replaced missing parts I would have contacted them as @gorc was missing a part for a tank tread in a kit he got from my parents ages ago and the not-quite-quite finished thing sat there taunting him for years (I think it was the last thing he needed to do) before he finally dismantled it to make room on his shelf so I think all the parts are in his lego tub now XD
They do and will send it to your door. It's great customer service. Google Lego and go to their customer service section and a link for missing parts. Easy as!