I remembered a oneshot I've read some time ago, I looked for it myself, but I can't find it.
I think it was oneshot in a volume with different main story or in collection of oneshots, so I wasn't able to find it using tags (funny story is, it's the 2nd time I forgot it, but first time I remembered what it was).
Protagonist was a high school student and he found a candy on his desk in school. Then he also left different one and started exchanging candy and notes with the person who left the candy. He found out the other person was a guy but didn't stop. The other guy didn't like strawberry candy because of his ex, I think.
Anyone remember what was a title of the manga this oneshot was in? 😀

After-School Strawberry Flavor, the third oneshot in Cello Mellow 🙂

That's it, thank you so much! 😀