Someone sent me one and I called SW to see about using it. She said, that unless my reward number shows her I am entitled to one, that I coudn't use it. I asked (nicely of course, since SW is always so nice to me) how others are using them, and she said once they give their reward numbers, it will pop up that they have a code. She said it has nothing to do with what another person gave them. She also said (and I have no idea if this is correct, it's just what she said) that anyone not entitled will not get the 20% off, and if someone calls and asks for the code to be applied, they will get it in any case when they give their reward number, but only if the computer tells her to give it to them.
Just some info..although I hope all that have gotten them from others are able to use them.