I ordered Turbotax software from Amazon. It arrived today but included in the package was a USB Flash Drive that I did not order nor did I pay for it. It was definitely a mistake on Amazon’s part because it was stuck in the packet of the Turbotax box..
Do I have to return it or does this fall under the category of טעות עכו”ם
According to what you are writing, since you did not order it and they sent it by mistake, it is their mistake and assuming that they aren’t Jewish there is no obligation to return it to them. You might want to make a Kiddush Hashem, and call them up explaining that you are Jewish and you were sent this flash drive by mistake, and that you are willing to give it back to them. They will not want to bother to take it back and you can keep it.
As a side point he Be’er Hagolah writes, “And I am writing this for posterity. I have seen many who grew rich from fooling others and they lost all of it, (as it says in Sefer Chasidim par. 1074). And I have seen many who returned the mistakes of goyim of large sums, and they became rich and left over a lot for their children”.