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

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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.
(1) Answer
Qualified Local Mercedes-Benz Shops
Qualified Mercedes-Benz Shops For This Repair