I had a problem with the post (actually it's customs) and I gift sent to me from mum
here
In the end, after several phone calls they agreed that I could send it back to customs for re-evaluation of the tax, provided I gave them permission to open the package (they had already opened it anyway so not sure why they then needed my permission) and there might be a small fee 5-10CHF I think.
In the end -and I have no idea how this worked out- it was sent back to me with absolutely no charges, tax or fee.
So, my advice to you would be call the number on the back of the docket from the post office. Don't listen if they tell you your only options are to pay the tax or send it back (which is free cos you can just "reject" it and it will be returned to sender). I got transferred to about 3 or 4 different people and in the end I think I was talking to poste customs at Zurich airport. Tell them you have proof you purchased it (hopefully you still do) and hopefully it will work out. Did most of this in English.
Also if it (or anything posted) is more than 6 months old and you can prove it you shouldn't have to pay the tax either.
Good luck!