If I ordered a set of 12 & they all came in separate cello wrappers I wouldn't be happy for a couple of reasons: 1) it's wasteful & not good for the environment. 2) what a pain to have to open all those wrappers!
How about using a strip of kraft paper wrapped round the middle to keep them all together, maybe with a custom made stamp, simple embellishment or moo logo sticker or address label to seal it.
I quite like the stationery box idea, but at the same time I can imagine a lot of people would just throw it out so it'd be wasteful. How about tying them with some ribbon and having a nice little tag tied on as well? Simple, not over-the-top but also pretty. :)
If I were sending invitations, I would find it irritating to have to take each one out of an individual bag as I prepared them for mailing. I'd say package them all together, however you decide to do so.