Can you take bicycles on post buses in Switzerland?

Yes, if there is space.

Check this link
http://www.postauto.ch/en/pag-startseite/pag-kundenservice/pag-velotransport/pag-nat-velo-und-postauto.pdf

I live in CH, but what's a post bus?? On the normal city buses (at least where I live) you can, they have spaces for them. On the metro too. You just have to make sure you go in the right entrance. Both the metros and busses will have a little bike picture on the best entrance for bikers. If that makes sense.....