The mitzvah /+ kidush hashem is far more then the $10 credit you are hoping for...
its also the DDF (i dont mean to go after everyone on DDF, only those guilty ones) mentality of, someone made a mistake, im gonna make them pay.
im not referring to price mistakes, although now that i write it out it can probably be debated on the ethics thread.
but if someone ships you 2 by mistake, and you paid for 1, return 1 at no inconvenience to you, and you dont have to be compensated for it. they made a mistake, its that simple. it didnt trouble you much. at least when there is a price mistake, you can rationalize that people are gonna buy them so why not me. (i have participated in several so im not just hear to be a critic)