Long story short:
I rented a car on booking.com because it offered a very competitive rate of $303 for 1 week in Miami. The rate was called an estimate and I understood what that meant: I expected 10% or maybe 20% of additional charges due to nickeling and diming. However, my final charge was over $700 by Thrifty.
I complained to Booking.com (I never use Thrifty, but I use Booking.com as my trusted agent all the time) and they gave me the run around: Agree this is a bad experience, contact Thrifty, nothing we can do. It turns out, I cannot even leave a review of my car rental experience on Booking.com -- they don't support reviews, but encouraged me to leave one with Thrifty.
Lesson learned: Never will I rent a car again on Booking.com -- do you agree?