One thing I know for certain is, never put FRAGILE stickers on your parcels. We were told by an ex-RM worker that such packages are deliberately thrown and sometimes dropped by po staff. Better to not put anything on the parcel, it has a wider survival rate.
The worse damage to a parcel I had was when I sent a walking stick through the mail. I actually put it inside one of those stiff fishing rod cases that I cut down to size? The thick heavy plastic tubes? It was also padded inside the tube with bubble wrap. Believe it or not, the stick arrived in five pieces! How on earth did they manage that???