dont pay another red cent. 17 year mechanic here. they r takin you for a ride/inept technicians. was the check engine light off when you picked it up the first time? is the coolant actually low? they could be charging you oodles just to clear a code off the cars computer. try removing the posotive cable from the battery for say...30 minutes. should clear the light. if it comes back they haven't been truthful with you. cylinder misfire is usually caused by a faulty plug/coil pack. any coolant leak would cause an overheat condition in the weather we have been having plus it would be in the oil if the coolant leaked from the headgasket into the cylinder.