Regarding all the comments about the middle seat: consider that, IMO, there's really not much wrong with a middle seat if all three passengers in the row fit completely within the width dimension of the seats. The middle seat has wider under-seat storage than does the aisle seat, so it has that as an advantage.
My wife and I both qualify as fitting completely within the width dimension of the seats, and neither of us much minds a middle seat with a third qualifying person in the row. The most recent version of our boarding strategy is to grab the closer to the front of either a) a pair of aisle seats or b) an aisle and middle next to a qualifying individual sitting alone at a window. We reconfigure if a qualifying individual claims a window seat in either of our rows after we have taken adjoining aisle seats with empty middle(s).
I can't see paying $5 for what the app says it offers, but I might pay that much or more for my wife and I to be assured seats in a row with a third qualifying individual.
It is grossly unfair that we often receive 3/4 of a seat for our money, while another individual in our row takes 1-1/4 seats for their money.