after visiting florence, i will stay in an agriturismo in siena for two nights then depart to milan, i've been searching for buses and i found one : www.sena.it however when i tried to book for the 15th may, it says cannot book for the particular date, up until now only the 30th april departures are available online, can someone help me to contact them (by phone?) and ask when will the online booking for 15th may available, i tried to email them and no reply so far (its been a month), i worry that maybe they will terminate the siena -milano route, as i will need to find other means of transportation and book early, i choose bus over train so i dont have to transit in florence, figure i will be in a lazy mood carrying luggage up and down 
) and it allows me to proceed with a May 15th date, letting me add a ticket to the cart and then asking payment method. I would say to try again, selecting the time of date you want and if you still have problems, let me know what you tried to see if it is the same for me!