Booking Pitti Palace and Medici Chapel is usually not necessary. I sugget booking the Uffizi museum instead where you might wait up 2 hours in linee before entering. Be sure to book from the official site http://www.b-ticket.com/b-ticket/uffizi/
Ticket costs: 11 euro + 4 euro booking fee. Non-official site charge much higher fees. Be aware.
www.villacastelletti.it almost 6 years ago