I've never heard about deliberate acts of vandalism in post offices.
However, judging from experience in Germany, part of the transport chain is fully automated, so that sticker or script means naught. And where things are manhandled, well, you should see the containers people send their stuff in... wrapped in newspapers (outside!), boxes that have a gazillion scraps from former destinations on them, product boxes and wine crates with photos on them, boxes covered in cutesy stickers...
So, postal workers tend to concentrate on the one important thing. The address label. So, just don't count on it they will handle anything different because of the fragile sticker. Take care of packing it as safely as possible.