Quick personal story. Had a small order of GH US dom TP. I think it was only like 5 kits. Got the package, then a few days later, got a notice that I had another package waiting. I was like hmm so I went down and it was another 5 kits.
I thought about it, not to say anything and probably nothing would have happened. However I had done business with TP for a long time and I didn't want to take the chance on that connection. I notified him and he was professional and I could tell he had delt with this type of thing many times before.
I offered to pay for the kits because 1. I could use them eventually 100%. 2. I had the funds. 3. I told him because it was his error, I would like to have the shipping charge waved and pay the 50+ kit int price or whatever the lowest possible bulk discount price he offered. He said that would be fair and so I paid him.
That was my specific circumstances and example. However others will vary.