I read the Love Letter extra as being something that is happening at the end of college. Whether they went to different colleges far apart, or Tsugumi went back to her brother's place for summer break to help her sister-in-law as she prepared for her new baby, I expect the two of them are going strong, but the fact that they live in a country where their relationship can never really be officially recognized makes both a bit uneasy...
Overall, I like the series.