It depends how they want to handle it. There was a thread a while ago about this similar thing. It totally depends upon who you talk to and how much they 'need' the convertible in MCO following your rental.

A few options that they are fully right in doing:
1. Just like Tuneman said... charge a one-way drop fee. They might add a mileage charge to it.
2. Tell you it's fine (in the event that the car happens to belong to TPA or somewhere other than MCO, they might get a higher cut of the final price).
3. Charge you the fees to retrieve it from TPA (a tow truck or 2 people's time to make the round trip, etc).
4. Flat out refuse to allow a TPA drop.

A few options: pick up somewhere other than MCO (Sanford airport or downtown location)? I know this might be a hassle, but it might be a lot cheaper if they have a car they're willing to give up; try companies other than Avis; or, make 2 rentals out of it (get a convertible from MCO and keep it for 2 weeks. Then drop it off and get a 1-way to TPA on the day you're flying out). They might not be the most efficient things, but sometimes you have to be creative if you have your heart set on a particular type of car.

Good Luck!