This was a very neat idea, especially for a week-long game jam!
I love the theme and the music - it definitely helped set up the fun concept of the narrative! I feel like mail delivery is an under-utilized concept in media and you clearly had a lot of fun with it.
I thought that the combat system definitely added more content to this particular submission, but I think that if you wanted to expand it, you could actually remove it and emphasize other traits of being a courier! As other comments stated, using more of your wits, making hard decisions, maybe leaning more into the visual novel or text adventure territory could be kind of neat?
I also wish that there had been some kind of a basic "How-To" or tutorial screen to help me get adjusted to the mechanics since I ended up wandering around for a few minutes without a letter to deliver lol. But I LOVE how traveling over certain terrain impacts your speed, sleep levels, etc. That was very very neat. It's no big deal since this is a game jam submission, but I also would've loved to see some more art for the monsters/player/etc.
Overall this was incredibly cool and I can definitely see it becoming fleshed out in the near future, although it was honestly very fun in the current stage!