Recently my dad received a package in the mail he did not order, yet the package contained stuff useful to my dad. My dad decided to keep the package that he did not pay for, even though the sender has now lost money. I know according to Canadian law he is allowed to keep the package but what is the Jewish ethical law on this case?
According to Jewish law he is not permitted to keep the package, and must try to return it to its owner.
The Torah specifies an instruction of hashavat avedah, returning lost property. The package is clearly lost to its owner, and therefore the obligation is incumbent to return it to it.
This can be done by contacting the sender, or by returning it to the post office.