I'm not going to contact amazon after receiving a refund on a product for which I didn't ask for a refund
Why don't you want to let them know about the mistake?
I am cheap and I want them to get more work done for me for free
Calling support to fix a problem that is not even your fault can be very annoying but it is the right thing to do. The money does not belong to you and they might take it back anyways.
Why? I'm pretty sure no one would notice.
It's wrong to not let Amazon know about the error. Keeping the money can backfire on you if they do fix the problem. You will then end up with less funds than you thought you had.