Interesting, the website does accept the underage coupon for bookings in April. It gives a red error message trying to also add the FL 1-way RQ, which it took fine last year. Perhaps after the big counter computer glitch that wouldn't allow the MIA agent to combine them for my reservation last year, Hertz has decided that the age group very likely to want this otherwise affordable roadtrip shouldn't have it, unless they agree to pay the normal off-peak very high rate.
If so, I hope Hertz reconsiders and reprograms their website, and counter computer system, to accept the underage PC and 1-way RQ like last year. While I won't be repeating the same roadtrip (that would be boring), I can recommend to those underage folks who might that the normal one-way rates into FL from NY can be lower than out of it, especially if you pick up the car from your friendly suburban NY/NJ/CT neighborhood HLE, the one who will likely be short on cars anyway and eagerly awaiting more to come in from FL. That would be counterproductive, yes, but I'm not the computer tech/policy maker.