to my understanding commercial endorsements are the best way for an artist to make money in Japan, since the cut for management is way less than in music/films/whatever?
are you sure that 100 million yen is the most anyone has ever got.
here's an article on Japan Today that states the actress/model/singer Yu Yamade gets 30-50 million yen per campaign, and although she is popular her celeb status is quite modest compared to Ayu.