We haven't planned to rent a car on this trip, but the train and/or bus trip makes me feel tired.
Pretty sure there are no rental agencies in Volterra
Try Pisa or Siena
AutoEurope.com see what they offer
I don’t know where you are staying, or from which city you are departing from and where you will go afterwards, but rental agencies with major rental companies tend to be only in the provincial capitals of Tuscany (and not even all) which are:
Florence (regional capital of Tuscany), Pisa, Lucca, Siena, Prato, Pistoia, Arezzo, Livorno, Grosseto, Massa e Carrara.
The only non provincial capital cities with major rental car presence are Chiusi, Viareggio, Follonica, due to proximity to major tourist destinations.