do this (and trust me, because i had to rent when we went to florida in july and i was 23)
go to travelocity or priceline, whichever william shatner does that lets you name a price
check prices of car size/class you want. you wont be able to get a caddy or lexus or anything luxury, but you can get most other cars.
when you go to put in your bid, it will say "if you are under 25, check this box, and understand there is a per day fee of XX dollars, so factor this into your bid".
well, me being new at it, i think the per day was like 23$, and the extra fee for under 25 was $25 a day. i put in a bid coming out to $40/day, or around 280+tax for the week.
now in retrospect, i should have checked the box, put in a bid of 3-4 bucks over the per day cost, and if they say no, go up another 2-3 bucks until they accept it.
shit, you could even try to check the box and bid what the normal rate is. what happens is, once they accept it, they cant hit you for any more fees, except tax. so if you need it for 7 days, put in a bid for $25/day, thats $175, make sure you check box saying youre under 25, and they can only charge you the $175 + any sales tax and insurance you decide to get.
you will need to give your CC info, they'll tell you about what the price will be, and submit your bid. they'll offer it to all the companies, and if one accepts your bid, it will charge your CC right then, and you'll get an email with all the confirmation stuff. if they dont, it'll tell you, then start over and up your bid a few bucks more