I just moved to a new little town. I know I could ask the postal worker, but I have social anxiety and he's so attractive lol

Anyways. There's one of the big blue mail boxes outside the post office and that's where I've been dropping my letters to mail out. This town doesn't get mail delivered to the houses, so we all have to have a PO box and pick up our mail. So I went in to get my mail today (after the employee left for the day) and noticed that in the door that leads to the back/ behind the counter, there's a mail slot that says "mail drop off" or "letter drop off" of something like that. So my question is, should I be dropping my letters in the blue box outside, or in the mail slot in the door? I assume it would make no difference, but what would the point be in having both?