Really liked this game. The music was awesome, the gameplay was pretty fun, it was a good idea to use the reject letter as the weapon. The enemies coming in felt very fair, even though it got a bit overwhelming in the last few, only having to get to 30 made it feel achievable.
I ran into a few issues, specifically scrolling on my laptop wasn't easy. It scrolls the whole window too, which was a little bit of a problem. And the [x to close] I thought I had to press the X key for a while, maybe it'd be better to have "Click here to close" or something.
Overall, I really liked it, and those leave particles while walking and after finishing the game were great. I also really enjoyed the music. Keep up the great work!