We find following all signs for Miami International Airport and then "Car Hire Returns" no problem whatsoever, more likely to make a mistake coming out of it, although we seem to have resolved this nowadays!

Sorry answered late last night and without my glasses on so didn't notice you were asking about coming in to Orlando regarding Dollar. However my comment about 2 international flights applies to either airport. It also depends what time of year it is - holidays will be much busier, so as long as you avoid the Memorial weekend (22 - 25 May)you should be ok. We made the mistake of travelling down one day without realising that it was a holiday and got caught in massive traffic jams making us rather late for checking in!