We have different mail people every day. I'm very lucky if I don't get other people's mail. I'm not going to subject them to a complex transaction.
These are the mailboxes. The letter is sitting up at the top but it's pretty loose and might not handle a good gust of wind. I've seen people leave letters like that but not very often. I certainly wouldn't do it with anything I didn't want to lose - which is most things I mail. Also mail theft/identity fraud is a real issue out here. I'm not gonna leave a check sitting like that all day.
The slot at the bottom is for bulk mail that won't fit in the mailboxes. It collects for days until someone throws it all out. They'll never see a letter in there.
This is a very common setup in CA. I haven't expected mail people to pick up my mail in most places I've lived out here. I usually just leave drop it off on the way to work or leave it in the office out bin. But I don't go into the office much anymore.