How to reset after service on a 99 C240?

See your owners manual. The instructions are in my manual. Be sure that you have the doors closed while you follow the procedure.

See your manual for the Flexible Service Schedule reset. It changes in some vehicles but it's something like:
Doors closed.
Turn the key to position 2.
Press the O (mileage reset) button twice.
Turn key to off.
Press and hold the O button while turning the key to position 2.
After about 10 seconds it resets.