In the United States we basically have two types of "official" translations, certified and notarized. (I put "official" in quotes because it is really more of the status quo, with the US government having no interest in regulating it.) ZipCar doesn't understand this anymore than you do, and that's why they write it that way.
Here are the accepted definitions:
Certified – This implies that a professional translator—meaning it is the person's profession, not just a native speaker—does the translation. At the bottom the translator signs a short statement saying "I am competent to translate from German to English and the foregoing is the complete and accurate translation of the original". The translator is expected to provide name, date, signature, and contact information.
Notarized – This is essentially the same as a certified translation, with one additional step. The translator signs the certification statement in front of a notary. Note that the notary's stamp and signature here are notarizing the certification, not the translation itself, because obviously the notary probably doesn't speak those languages!
Technically it is not allowed by the definition above, but you could have your friend do it if the person follows the instructions above. Ultimately, ZipCar is the only entity that will decide whether your translation is "official" in their eyes. Interestingly, government organizations like the USCIS (immigration) only ask for certified translations these days, but they do insist on a professional translator, not a friend or relative.
My suggestion is to find a local translator (do a Google search). They should charge you less than ZipCar's annual fee.