I have posted a few items for guests (I do live close) and if I wasn't close by, I would still try and arrange it
And that is lovely, and excellent customer service, and well done you. Definitely how we should all behave towards each other.
However, not going that extra mile does not automatically equate to being something worthy of complaint!
I have had many guests who took the sheets off their beds, put the recycling into the correct bins, left me a gift and a thank you note on the table. I appreciate it, and think they are lovely people. But they didn't need to do any of it.
Plenty of other guests don't do any of those things, even leave things kind of grubby. I'm not talking about terrible, not trashed, just not as good condition as some guests leave it. And it's fine, because I expect to check it and clean after them. I wouldn't fine them or write them a bad review for not being exceptionally thoughtful human beings.
Re returning lost property, sometimes it's easy, sometimes it's not. Some Airbnbs and hotels operate on a case by case basis, some see it as an opportunity for extra earnings. It's great if they are generous, but you can't assume that they will be.