You only pay to park once a day. So if you go to the MK first and then to AK, you just show your receipt from the first parking lot and they will let you in for nothing extra.
DTD parking is for guests who are shopping, dining or taking part in other entertainment at DTD only.
They discontinued the buses to the parks from DTD and back because all the people trying to avoid the parking fees of the theme park lots.
If you have an annual pass, you can get free parking. If you stay at a Disney owned resort, you can get free parking.