In our experiences, the rental companies usually only have "regular" car seats...the toddler kind. They add on $5 per day with a maximum charge of $40-50. Our little one really prefers the boosters, so we invested in a carrier for it (toys R US/ babies r us carries them). It only cost $20 and we use the extra space in the bag to all our goodies on the flight home.
We figured that instead of paying the extra $5/day for 7 days, it was just as easy to put our booster seat in a big plastic garbage bag and check it through with our luggage. We had no problem flying to or from MCO and our car seat came through just fine.