After more than 8months in Japan, I finally found out today from a colleague that we can actually buy Coupon tickets (回数券, KaiSuuKen). You basically buy 10 tickets of a fix price (example: 160yen, 190yen) and get 1 free. so 11 pieces of 160yen tickets cost 1600yen.
what’s the coupon for? train??
yeh. this is for Tokyo metro line