What is service 'D'? on 2006 Mercedes-Benz R350

We bought this car three month ago with mileage about 64K.
About month ago we find out that it needs service 'D'.
We tried to find over the internet what is this and how much it cost. At least to know what to be ready for.
Is this a good idea to safe some money and do it and other then MB shop?
We appreciate any information.

Asked by for the 2006 Mercedes-Benz R350