1999 Jeep Laredo tune up

Engine Mechanical problem
1999 Jeep Laredo 6 cyl Four Wheel Drive Automatic

I reecntly brought in my jeep to be serviced for thee check engine light. The mechanic found serveral problems and told me in the future I was going to need a tune up because the engine is shaking and the car has little power. Around 3 weeks later I bring the car back and the tune up is going to cost about $250 which I thought is expensive considering he said oil change is not part of it and fuel injection cleaning is not part of it either. What is he changing that is so expensive? Just the spark plugs? The car doesn''t have wires like my older cars used to?Also if he does change the spark plugs how can I check and make sure he did what he said he was going to do?