You can find different-sized boxes at Coop Bau & Hobby or OBI. They're often called moving boxes, but if taped up well, you can also use them for shipping.
I agree that shipping via the Post is probably the cheapest. I think you can check the shipping costs via the Post on their website: http://www.swisspost.ch/post-startse...t-priority.htm
It says: "The maximum weight for PostPac International Priority packages is 30 kg and 20 kg for some countries. Your consignment must always be signed for by the addressee."
Oh, here's their page for the shipping rate calculator: http://www.post.ch/en/post-startseit...detail_eingabe