There is one 370A bus on Sunday from Florence to San Casciano and it leaves at 9am.
Here's the schedule:
https://www.at-bus.it/media/mod_timetables/timetables/EXFI/370_A.pdf
There is another bus line that also has a bus on Sunday between Florence and San Casciano on Sundays at 13.30 (bus line 368A):
https://www.at-bus.it/media/mod_timetables/timetables/EXFI/368_A.pdf
The columns with
E7G at the top are the ones that run on Sundays from June 12 through the 30 for both those buses.
Are you arriving before the end of June? You said two weeks and a Sunday but it isn't clear whether you mean June 25 or July 2.
It is possible that the line will continue in July but the schedule gives those precise dates for those buses. So we have to see whether at the end of the month they update the schedule with hours for July... or they don't and it means that there won't be a bus on Sunday in July.
Bus schedules are hard to find on that site! They are on this page:
https://www.at-bus.it/it/orari.html. You need to click on "extraurbano" first, then wait for some links to appear below, then click on "extraurbano FI" and then wait for all the lines to appear to the right. That is a list of buses for the area around Florence.
If you're using buses to move around during your stat, know that there is another bus line that travel between San Casciano and Florence - just not on Sunday. Line 371A:
https://www.at-bus.it/media/mod_timetables/timetables/EXFI/371_A.pdf
Hope this info is useful!!