.my comment stems from 2 buyers who were really nasty no matter what I tried to resolve the situation.

There is only one thing you should do, ask for a photo of the broken item and once you see it you refund in full. Anything else and of course the buyer is going to turn nasty.

Your items need to be packaged well enough so they can be dropped from shoulder height onto a hard surface and they also need to withstand packages of up to 20kg being dropped on top of them.