Oh, that's right... I knew your name was familiar but I couldn't remember why. The good thing about going into MCO is that the convertible selection is much better than TPA!

Just so you know, Hertz is off-site at Orlando airport. I've used them before and they're a lot easier than most off-site locations at US airports, but still not as convenient as the on-site options. A thread about that is here, if you want others opinions.

A few other option that you might try, if you really want to use Avis and definitively want a convertible:
1. Book a full-size or other car at Avis & sign up for Preferred service. When you get there, try to persuade them into giving you a convertible. In this case, I'd have a back-up reservation somewhere else in case they won't upgrade you.
2. Book the convertible from Hertz or other location and print out your reservation. When you get to MCO, go to Avis and ask them to match it. The rental companies are very hard-up for business right now and on-site agents also get a cut of local sales. Even if they refuse, you can walk (or bus it) to your original reservation.

You can try a phone call, but usually you have much better luck in-person. If you have any other questions, don't hesitate to ask; that's what we're here for. Enjoy your stay in FL... I'm there again next week (TPA) and can't wait.