Photographs before the conversation, always. The instant a contractor knows there is a dispute, the packaging goes, the offcuts go, and what is left is your word against theirs.
Photograph the delivery when it arrives, not just the finished work. A photo of a pallet with a label on it, dated, is the single most useful piece of evidence in this whole category and it takes five seconds while the delivery is happening.
I now do this on every job as a matter of routine, and twice it has ended a disagreement in one message.