Florence to Volterra by bus or train

I'm sorry, but traveling down to Volterra from Florence by bus or train is just complicated.
Viator possibly has a well priced tour going there:
https://www.viator.com/tours/Florence/Volterra-and-San-Gimignano-Tour-with-Vernaccia-and-Cheese-Tasting-from-Florence/d519-5919P228

If it must be a Sunday, don't attempt to go by public transportation, rather rent a car or find a private tour as David pointed out in his comment. There is very little bus service on Sundays.

If it can be on any other day, there are two main options with public transportation:
OPTION 1:
Bus 131 from Florence to Colle Val D'Elsa (see below)
http://www.tiemmespa.it/index.php/Viaggia-con-noi/Orari-e-linee/Siena/Extraurbano
then from Colle Val D'Elsa you would switch to another bus (#770) from Colle V.E. to Volterra
http://www.pisa.cttnord.it/Linee_e_Orari/Linee_Extraurbane/L/134

OPTION 2;
Train from Florence (Firenze) to Pontedera
www.trenitalia.com
Then bus #500 from Pontedera to Volterra (see the second link in option 1 and select bus 500.

I would choose Option 2.

Be aware that the bus timetable linked above appears to be the winter timetable. The summer timetable should become available shortly in the next week or so.

I went to Volterra via Pontedera and returned via Colle Val d'Elsa (no need for the extra change in San Gimignano). I thought there was very little difference between the routes, and the trip wasn't at all difficult. As previously mentioned, there are very few buses on Sundays. The winter schedules have only one Sunday bus per route and they run late afternoon or early evening. The trip would be much easier if you could change your travel day to Saturday or Monday. Avoid buses marked "S" on the schedules, as they'll be jammed with school students and you might have to stand for part of the trip.

If going via Pontedera, it's about a one block walk from the train station to the bus station. I bought my bus ticket at a café adjacent to the bus stop. If going via Colle Val d'Elsa, I seem to recall the buses arrive and depart on opposite sides of the street. Ask a local to be sure. Tickets can be purchased at the kiosk adjacent to the bus stop if needed. The trip involves 2 different bus companies and I had to buy the 2 tickets separately.

EDIT: I looked at the notes on the CPT website, and it appears no separate summer schedule will be posted. The winter schedule is in effect for September, except that the school (S) buses don't run before September 15. That means just one Sunday bus to Volterra on each of the 2 routes in September.

Thank you everyone!

Hi David, did you by any chance take that tour? I am also looking at it as an option.

Thanks