If you want a grace period, as I wrote in the third post of this thread, you have the option to request a prorroga de permanencia which will extend a 90 day visa by 90 days and there is a grace period of 30 days after the 90 day visa expires to ask for the prorroga.
Of course you would have to pay for the prorroga, but the $2700 peso cost of the prorroga, if applied for before March 20, is $1800 less than the cost of the $4500 (in most cases) overstay fine. I beleive the prorroga will cost $4050 if requested duting the 30 day "grace" period.