lol at the author's note. Cookies usually aren't an attack, especially from a supposed friend!
You would be surprised.
(Something something about two nickles. Although, I wonder if it's because of culture norms in Japan? because baking might express feminity and such? Because it's expected from a "perfect" housewife?)
I feel like Rio's mindset might have been "I can bake these cookies for all of you, with such delicacy, and exquisiteness... but I bet for sure you cannot even cook as much as that, my friends... especially you, Mahoro~". Like, feeding your own ego and the like, an act of superiority towards her friends - the always single Momoka, the "married but zero sex life!" Kyouko and "failure wifey with a perfect marriage" Mahoro.
And it's kind of sad? Because I think the others were all simply like "yay! cookies! expected from a friend with her own café!" kind of way?
And since Mahoro didn't react negatively or threaten or whatever... maybe that's why Rio ended up going to the next step with the letter and the used condom? in a "lmao, I'm having SO much better sex with your husband than you have!" way (although I feel Rio setting up the SNS account was probably the step before that).