"What happened to my letter?"
Never, never ask a guy to take a letter to the post office, and simply put the letter in the mail slot. You're just asking for trouble. If you do ask, you're on your own; don't say I didn't warn you. They don't mean to screw up mailing a letter, but in my experience, with several different individuals, they just will. They can't help it. It's their duty, because they care about you, and think they care more about your mail than you do. They need to do the Right thing.
You can't blame a guy for screwing up dropping a letter in a slot--he just doesn't get that that's exactly what you actually want him to do, and he will do something else. He may carry your letter around inside his jacket for a few days or weeks, guarding it safely where no one can mess with his precious cargo.
He may happen to see an official-looking abandoned street mailbox that's a lot closer than the post office, and therefore faster, so that he can be home to you sooner; and he will drop your piece into that dark steel pit where it will get picked up in about a week. Or six. Or never.
He may decide your letter looks important, and send your tax check by certified mail, where it gains the Internal Revenue Service's attention, prompting an investigation.
You certainly shouldn't tell a guy that a letter's important, or else he may decide to send it by registered mail. You'll find out when you get a strange letter from the recipient's attorney in about ten days.
And definitely, definitely do not hand your letter to a postal worker who says, "I can just drop this off to the recipient on my way home; I go right by there." Because he will think that the corporate initials on the front of the envelope stand for "Boston Police Department;" and even though he is a dutiful employee of the United States Postal Service, he has blind devotion to all authority; and if he thinks something is supposed to go to the police, he will dutifully, without fail, and with great care and concern, take it there immediately. Without reading the address. And the check that you thought you rushed to your landlord will float around inside the police station for two months. And you will get evicted.
There is simply no way of telling what any man will do with a letter when you ask him, "When you go by the post office, would you please put this in the mail slot?" Trust me. You can't hold him responsible. He's absolutely sure that he knows better than you what should be done with your mail, and you have no control over that. He can't help imself. Let it go. Send your own mail.