There is no reason to bring a ski jacket. Simply bring layers as others have suggested. I'm leaving tomorrow for 16 days hiking in/around Wengen area. I'm taking t-shirts, a lightweight stretch fleece long sleeved top, a light vest, a windbreaker, and a lightweight Gore-Tex jacket with a hood. That rig will see you through anything you will encounter in August and September.
There is a Post Office right in the center of town. You can mail things to/from there.
However, we have only mailed things back from there and both times the package never made it. Swiss Post is highly reliable. We believe that the packages were stolen by US Post employees once they entered the US. Both packages included new items (a Rolex watch box, box only of course! and expensive hiking pants that were intended as a gift). Perhaps if you are shipping used clothing and shoes you won't have a problem.
Another option is Luggage Free http://www.luggagefree.com. Highly reliable but very expensive. Like $250 or so each way for a medium duffle bag.