Most companies will charge the underage fee - in fact, i dont know of a company that won't charge this fee. True, Hertz has a temporary promotion for the Christmas holidays where they are waiving this fee, but don't expect to see this on a regular basis. It's just a promotion.
There is one company that will rent even to those under 21, with a fee of course. They have more flexible guidelines. Locations in florida include orlando, miami, tampa and west palm beach. Others including atlanta, dallas and lax.
Learn more here: ez-rentacarusa.com