According to your service manual the A & B service indicators alternate every 10,000 miles. They are mostly fluid and filter changes & one or two other things; which frankly you could do yourself. It seems your reminder kicked in at 18,000 miles & will count down till you reach you 20,000 limit then will remind you every time you turn on the engine and will start counting how many miles over the 20,000 you have done.
When you had the oil changed, was the computer reset to count another 10,000 miles? That seems to me where you made the error; that oil change was unnecessary. You are uning Mobil 1 Synthetic 0W 40 European Formula (I think that is the correct spec, I'm not in my garage)
For my C230 Kompressor, a B service is about $300.
2004 C230 Kompressor SS Black, sunoof package, heated seats, CD changer, custom door pin surrounds & pins, clear side markers & 5-speed Automatic