What a weirdly heavy detail to make up for a single, smooth one-liner interaction between our main couple in a yuri fluff manga. I guess the author got the idea for "lost mail" at some point and decided to dedicate a chapter to it, and the only realistic set up was the mother being a horrible, bitter person who could have very much ruined her child's relationship with her best friend. Not even "could have" actually, she did. Before they met back up in high school, the catalyst for the falling out of their relationship was absolutely her mother continually snipping the lines of communication between the two. That's fucked, even more so to hide that fact from your daughter all these years. I guess she's not going to be relevant to the story much so it doesn't matter.