I'm in the middle of reuniting Kafei and Anju, but I can't give the postman the letter. it won't let me into his office/home thing, and it's the final day. when i restart by playing the song of time, it restarts everything and i have to get the letter from Anju and so on all over again. how do i get it to the post dude in time?

Listen!...
Which letter? The one to Kafei, or the one to his mom?
If it's the one to Kafei, you just need to put it in the postbox. If it's for his mom, you can deliver it yourself, if you have Romani's mask! :3 She's in the milk bar.