You're contradicting yourself- is this a return transfer or is this a roundtrip transfer? I'm going to assume it's roundtrip (airport to wdw to airport).
You tip the driver each direction, this is because you will most likely have different drivers. Customary tip for private towncars is around 20%. So, if the roundtrip total is $207, then each direction is approx $100, 20% of $100 is $20 so your math is solid. $20 each direction.