You can send your passport renewal by mail either with Deutsche Post Einschreiben Einwurf or DHL ExpressEasy. I would advise anyone thinking about this to NOT use Einschreiben Einwurf as they lost my passport and have advised me it will take more than a month before I can even get a loss confirmation so I can apply for a new one. It's a nightmare, as the Jugendamt is trying to remove my kid from the school system because my passport expires in June.
EDIT: Although the post says the letter wasn't delivered, the embassy wrote to tell me it actually was delivered. In any case not a good method to send given the importance of delivery.