I don't disagree with Kendall often. However...
The copper washers MUST crush "a little" to seal the plug. The trick to reusing them, whether they came from the Ford parts dept. or Ace hardware, is to not tighten them any more than necessary to seal the oil, and certainly not any more than the torque specification (otherwise you may damage the threads in the aluminum pan or tank). They are both annealed soft copper, or in some other applications, aluminum. If you don't "crush" them enough that you can see them deform, then you can reuse them several times. If you have to tighten them to the point where they are visibly squishing, and not sealing, then it's time for a new one. I also buy them from Ace or Wilco.