[To get one from his friends is natural. To get one from one of his "dads" is to be expected. But Kasumi's one takes him by surprise, both because it shows that she remembers him despite only having talked once and because, well, as far as he knows the macarons are limited which means that either she doesn't know many people, or that out of everyone she knows she consciously decided to give him one.

There's also the option that she wouldn't trust the macarons and would be sending them to people she doesn't know much in order to avoid feeling too bad if there are negative effects. But for that it would make more sense to just drop them in random mailboxes, not go and write a note and everything. At any rate, after pondering about it for a while, Yoh eats the macaron and sends her a reply with one of his own.]

Kasumi,

thank you for the macaron! It was delicious and a nice surprise to find in the mailbox. I'm sending you one of mine, I hope you can enjoy it as well if you're not tired of getting them yet.

Yoh Asakura