There is a shuttle service from Miami to Orlando, but the reviews I've read on them were extremely poor. So, even if the cost is less with them, I wouldn't use them.
More expensive than a rental car, but there are flights from Miami to MCO. You could take a cab to the airport in Miami (not far and I think it is a fixed rate of about 20$ or so) and then Magical Express to your WDW resort.
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