The only way to drain the torque converter would be to remove the transmission and torque converter. None of our cars have the drain plug on the converter. The other way to get a complete flush of the fluid would be to use a specialized machine. For $150 they probably just drained the fluid in the pan and replaced the filter. About half of the fluid is replaced when doing this(about 3-4liters). If you are certain they did a complete flush (with a machine) please let us know who is willing to do this for so cheap.