If the car is 6 months old the letter will be about an oil change "interim" service surely. Many new cars have a 1 month or 1000-1500km check up, but you have well exceeded this time period.

Im guessing that VW is trying to sell you a 6 month oil change which although good to have fresh oil in the engine, will not count as a regular 15,000km / 12 month service.