Please tell us how this goes! I've been renting cars in Mexico for over 15 years and never saw anything like that. I hope I've been wrong all this time and will save hundreds and hundreds of dollars in January and February of next year.
I believe in the "No Free Lunch" theory. Four days for $12 including insurance? I can drive a car for $90 a month plus the gas and they pay all other expenses and maintainance? Why would anybody own a car? And if the car costs the rental company $12000 they can have it paid for in only 11 years, although their tires, oil changes, liability insurance, and interest might lengthen that out a little. If it is too good to be true then it isn't true! That said please, please tell us how it goes!