Most resorts make you pay for parking, Disney doesn't. When you rent a car in the US, the gas tank is full. You have to return it with a full tank, otherwise you're charged to fill it, at a price per gallon the car rental company sets. Fill up the tank before you return the car. Some companies may charge a relocation fee if you return the car to a different location. I doubt that will happen for Miami - lots of travel between Orlando and Miami. But all this you can verify with the rental car agency. And yes, there will be taxes, but nothing near what you would pay in Canada. One other thing, US car rental companies won't rent to drivers under 25.
It's an easy drive from Orlando to Miami. You just take the Florida Turnpike. It's a toll road, so be prepared to stop and pay a couple of dollars each toll plaza.
@Figment82, does your "Uber" driver remember if they charged you to park at either Epcot or TTC for the races - or do they just wave you through?