Yeah, mistakes happen. Never had a really bad one, where they didn't want it back but wanted me to pay for it.
Once I ordered a set of dies for my Pressmaster crimpers from TME, and received a pack of 5. These are 35-40€ a piece. I called them and they apologized and thanked and asked (very politely) if they could ask me favor to return surplus if they send a courier to get it. I sent 4 back, and that same lady even thanked me few days later when she received it.
Fast forward, I ordered few more dies few months later. I receive them but there was one additional type I didn't order.
I repeat the routine, report to them. Same lady sends an E-mail, says hello, apologizes for making mistake again, (she remembers previous episode) and asks me bluntly, do I have a use for the die? She says, if I find it useful, she would be glad for me to keep it. So they left it with me.
As fate would have it, after a month or so, that particular die came in handy on one job...