I'll have to look for this yellow box at my post office, where you can throw in any ready labelled packets. That sounds good.

I've just looked the the standard range of "PostPac" boxes which are on sale at their shop. The smallest one available is just 3cm too wide to qualify for their "small consignment" rate. ( I suppose they will not be in a rush to make smaller ones available)
https://www.postshop.ch/shop/de/fran...oko-0/p/730834