Carnation is infamous for including unexpected and unneeded charges in the bill (as usual in case of a workshop). Please be careful in future while dealing with them and clearly inform that no other items should be included in the bill without your prior consent.
Always check with the manufacturer if the engine requires additives. Do not spend money on additives. Engines are designed to run without additives (unless the manual tells you to add extra stuff to keep it running).
Also check if putting additives in engines could affect the warranty.