I have mixed feelings about this game.
On one hand, you've just taken graphical elements from other games and thrown them together which is mildly annoying in itself, it makes the game feel lazy and novelty, I actually even wonder if you did some of them yourself or if you just patched together sprites directly from the games, because the characters don't exactly fit in with the scenery. Then you get this comment from the character saying something like "lol the creator of this game must be so creative lolol", must you really plug yourself like that? The whole thing is full of references to other stuff to the point that I feel like the only reason this exists is to be a reference to other games. It's one thing to try and create a retro, familiar feel to your work and another thing entirely to make something that just looks directly copied.
On the other hand, there must have been something that kept me playing until the end and further through the "secret" poster. I can't really complain about gameplay or controls, and I tend to like games where the ultimate mission is just to accomplish a really mundane task like delivering a letter. It's a nice change from being the chosen one who has to save the world, so ignoring how annoyed I was with your theming it's actually a pretty cute little game that I enjoyed.