Otona-chan.
This was sooooo cute! I got diabetes! 3 stars because apparently I've read this recently and forgot to mark it as read. I didn't notice until the last few pages of ch2/first story ;; That's honestly the only part I remember for the first story along with the extra chapter. The second story is also a bit forgettable. I remember chapter 3, but not the others ;;
Magic Mail Chocolate
The first story really makes you beg for more, but at the same time, it ended at the perfect moment. Second story is meh; cliche and just... that. Third story is painful, 2 parallel unrequited loves; again, like the first story, makes you beg for more, but ended at the perfect moment. Last story is the best in terms of length though; unlike 1 and 3, it has an actual conclusion.
K-sensei no Yajuu na Aijou