I have to give MC credit. People are ragging on him for giving her umeboshi, but I believe it was the closest he'll come to an act of genius in this lifetime.
Sumi-chan's advice was to give her something to convey his true feelings. Can you think of
anything that says "Kazuya's true feelings" better than a box of ridiculously-salty shriveled red balls?
(image of umeboshi, not a spoiler)